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Dark Heart: A True Story of Sex, Manipulation, and Murder
From the authors of the “true crime classic”* Notes on a Killing comes the harrowing story of the games that couples play—and what happens when role-playing becomes a deadly reality.
Was murder part of the game?
Seventeen-year-old Kat McDonough grew up with theater in her blood—and a penchant for make-believe. More than a decade older, Seth Mazzaglia was well known in the community theater circle of Portsmouth, New Hampshire. He, too, had a rich inner life—and he soon had Kat convinced that they were soul mates.
To intensify their bond, Seth lured Kat into a world of violent sex and role-playing, where she was his slave. But even that wasn’t enough to satisfy his ravenous appetites.
Enter Lizzi Marriott, the new girl in town. And when she accepts an invitation to Kat and Seth’s apartment, she will never be seen again...
INCLUDES PHOTOS
*Robert Scott, author of The Girl in the Leaves
Was murder part of the game?
Seventeen-year-old Kat McDonough grew up with theater in her blood—and a penchant for make-believe. More than a decade older, Seth Mazzaglia was well known in the community theater circle of Portsmouth, New Hampshire. He, too, had a rich inner life—and he soon had Kat convinced that they were soul mates.
To intensify their bond, Seth lured Kat into a world of violent sex and role-playing, where she was his slave. But even that wasn’t enough to satisfy his ravenous appetites.
Enter Lizzi Marriott, the new girl in town. And when she accepts an invitation to Kat and Seth’s apartment, she will never be seen again...
INCLUDES PHOTOS
*Robert Scott, author of The Girl in the Leaves
384 pages, Paperback
Published March 1, 2016
38 people are currently reading
267 people want to read
About the author
Kevin Flynn
7 books110 followersKevin Flynn is the author of four true crime books: "Wicked Intentions," "Our Little Secret," "Legally Dead," and "Notes on a Killing," as well as the nonfiction book, "American Sweepstakes." He has appeared on such TV shows as "Unusual Suspects," "Deadly Women," "Killer Instinct," "Deadly Sins," "Pretty Bad Girls," "Nightmare Next Door," and "On the Case with Paula Zahn." He has also written for such publications as Playboy and playboy.com.
Kevin's primary writing partner is his wife, best-selling author and public radio producer Rebecca Lavoie.
Kevin is a native of Holyoke, MA. He got his start in journalism at the age of 19 working as a news writer for News Radio WHYN-AM in 1989. In the subsequent twenty years, Kevin has become an award-winning journalist and respected author.
He rose through the ranks of WZID-FM in Manchester, NH, starting off at 8 hours a week until he eventually became News Director. During his decade at WZID, he won more than 50 awards for broadcast journalism, including multiple New Hampshire AP awards, several Golden Mike awards from the New Hampshire Association of Broadcasters, and four Edward R. Murrow Awards from the Radio and Television News Directors Association. The award winning pieces covered a diverse set of topics, including features on gang intervention, secret government meetings, opening day at Fenway Park, and a visit to Ground Zero in New York City. Kevin was also three-time winner of the AP Contributor of the Year.
In 2002, Kevin joined the staff of WMUR-TV as a general assignment reporter, again winning several honors for himself and the station. For the first feature piece he ever did for television, Kevin won a New England Emmy Award - the first reporter from his station ever to do so. Kevin excelled at both hard news and soft news features. He covered deadly floods, the murder of a police officer, and presidential politics as easily as his fun stories about spelling bees, karaoke, and dodge ball. He was named "Best TV Reporter" three times by the readers of New Hampshire magazine.
Kevin's primary writing partner is his wife, best-selling author and public radio producer Rebecca Lavoie.
Kevin is a native of Holyoke, MA. He got his start in journalism at the age of 19 working as a news writer for News Radio WHYN-AM in 1989. In the subsequent twenty years, Kevin has become an award-winning journalist and respected author.
He rose through the ranks of WZID-FM in Manchester, NH, starting off at 8 hours a week until he eventually became News Director. During his decade at WZID, he won more than 50 awards for broadcast journalism, including multiple New Hampshire AP awards, several Golden Mike awards from the New Hampshire Association of Broadcasters, and four Edward R. Murrow Awards from the Radio and Television News Directors Association. The award winning pieces covered a diverse set of topics, including features on gang intervention, secret government meetings, opening day at Fenway Park, and a visit to Ground Zero in New York City. Kevin was also three-time winner of the AP Contributor of the Year.
In 2002, Kevin joined the staff of WMUR-TV as a general assignment reporter, again winning several honors for himself and the station. For the first feature piece he ever did for television, Kevin won a New England Emmy Award - the first reporter from his station ever to do so. Kevin excelled at both hard news and soft news features. He covered deadly floods, the murder of a police officer, and presidential politics as easily as his fun stories about spelling bees, karaoke, and dodge ball. He was named "Best TV Reporter" three times by the readers of New Hampshire magazine.
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Displaying 1 - 30 of 44 reviews
June 9, 2017
There was something missing throughout the whole book. It feels as if some information was not put in. I honestly do not believe that Lizzie went to a couple's house and decided to play a game of three of strip poker. First and foremost she had "stated" that she was in a very committed relationship with Brittany. So why play the strip poker? Also, Kat lied a whole lot in and out of court, so if feels as if the story has some holes. First it was the position in which they were apparently having sex. Then it was the a rope. They went with the rope and finished the case. Also, first Lizzie was apparently into it, and then she was not into it. I think there should have been more conclusive information before the trail begun. There should have been more interviews with friends rather than family. The family would know Lizzie, but not truly know her sexual fantasies. It is more likely that friends would know. Then again, Kat had very violent sexual fantasies in which she wanted people to know about. Here and there should would mention how bondage was her everyday life. That should have struck Lizzie. Anyway, the book was a bit boring because of Darkheart and Skarlet and the other personas that kept popping up. It just seemed that the whole book was about Kat and Seth's lifestyle of bondage S&M. There was one like two chapters in the beginning about Lizzie and then we had to wait until she appeared again while working together with Kat at Target. The fact that Seth had a very cruel and abusive way of showing love that completely fell apart when he was being interviewed was also weird. He admitted to the crime and decided to protect Kat. Perhaps he had another persona that was actually nice?
December 18, 2020
Mini Review I’ve been on a true crime kick lately and because I am familiar with this case, I decided to give the book a try. Firstly, the book (and the case) is interesting. The authors do a good job at giving us inside details of the perpetrators and how the crime happened. HOWEVER, there wasn’t enough information on the VICTIM. The crime that befell her was almost an afterthought to the book because so much of it surrounded the two killers. It didn’t work. The focus in a true crime book should be directly about the CRIME that happened not 250+ pages about the killers and then a little bit about the victim and trial. Had this book not kept me intrigued from beginning to end (these killers are sick and twisted and it was hard NOT to read about their lives and what lead up to the murder) I would have given it even less stars than the 3 I’m giving now.
July 25, 2021
This true stuff is unbelievable!!
May 19, 2016
This is the first time I am totally not enjoying a book written by Kevin Flynn. Why is that you ask?
Well because even though I do not know what is going to happen, it is already very obvious which of the 2 had a close connection with the authors. Talking about being biased. My God . Not even sure I can read any further to be honest.
Everything the male does is being looked upon as negative but everything the girl called Kat does is being marginalized or it is his fault.
It has gotten to the point that I am not sure I can even read any further. Kat lies but there are constant excuses for that. It is all his fault.
What is wrong? I understand that an author can get a connection with (I assume) a criminal but then to constantly defend all her actions and belittle his as if the reader cannot form their own opinion is in my eyes one of the worst things an author can do while writing a book.
Please Kevin Flynn Don't make that mistake again pretty please.. Loved all the other books I have read. Not so long ago I gave this author huge compliments in another review I wrote on Goodreads and I will keep buying his books.
Well because even though I do not know what is going to happen, it is already very obvious which of the 2 had a close connection with the authors. Talking about being biased. My God . Not even sure I can read any further to be honest.
Everything the male does is being looked upon as negative but everything the girl called Kat does is being marginalized or it is his fault.
It has gotten to the point that I am not sure I can even read any further. Kat lies but there are constant excuses for that. It is all his fault.
What is wrong? I understand that an author can get a connection with (I assume) a criminal but then to constantly defend all her actions and belittle his as if the reader cannot form their own opinion is in my eyes one of the worst things an author can do while writing a book.
Please Kevin Flynn Don't make that mistake again pretty please.. Loved all the other books I have read. Not so long ago I gave this author huge compliments in another review I wrote on Goodreads and I will keep buying his books.
This entire review has been hidden because of spoilers.
August 21, 2019
Oh man. I do like an occasional True Crime novel but this one was to much for me.
The abusive relationship is shown here and the writer spares us nothing. It is a tragic story as really all True Crime is but this book goes into graphic details on some things that made me very uncomfortable.
My heart does go out to the victim and her family and friends. It will leave you a bit shaken, n o doubt about that.
The abusive relationship is shown here and the writer spares us nothing. It is a tragic story as really all True Crime is but this book goes into graphic details on some things that made me very uncomfortable.
My heart does go out to the victim and her family and friends. It will leave you a bit shaken, n o doubt about that.
March 22, 2023
Heard about this case on the podcast True Crime Campfire and wanted to learn more.
Something about Lizzie's story really struck a chord with me, maybe because she was only a year younger than me and had a lot of similar interests and hobbies. She could have been my friend had our paths ever crossed. I almost cried multiple times listening to the audio book, especially during her candlelight vigil when they played a clip of her singing Into the West from Return of the King. What a beautiful soul who deserves to be remembered for the passionate and sunny individual she was.
Also, important to learn about, are the dangers of domestic violence and the depths of mental and sexual abuse Kat suffered at the hands of Seth. Kat is by no means innocent, but I think her story needs to be heard, as a warning to young girls and parents alike.
Very informative and easy to listen to audio book, I would recommend this to anyone with an interest in true crime, but definitely not for the squeamish.
Something about Lizzie's story really struck a chord with me, maybe because she was only a year younger than me and had a lot of similar interests and hobbies. She could have been my friend had our paths ever crossed. I almost cried multiple times listening to the audio book, especially during her candlelight vigil when they played a clip of her singing Into the West from Return of the King. What a beautiful soul who deserves to be remembered for the passionate and sunny individual she was.
Also, important to learn about, are the dangers of domestic violence and the depths of mental and sexual abuse Kat suffered at the hands of Seth. Kat is by no means innocent, but I think her story needs to be heard, as a warning to young girls and parents alike.
Very informative and easy to listen to audio book, I would recommend this to anyone with an interest in true crime, but definitely not for the squeamish.
March 2, 2016
Couldn't put this one down. So much manipulation and deception! I do feel like there's so much more to Lizzie's story that was not in the book, most likely in respect to her family's wishes. Kat is the fascinating one. I'm still not sure which of her stories to believe or whether to feel sorry for her or blame her. I do feel like there was more to her childhood that wasn't revealed in the book and we are left with a confusing story where that's concerned. Overall, a good book about a horrifying crime.
March 12, 2016
I'm not much of a true crime follower, but I'm a huge Rebecca Lavoie and Kevin Flynn follower, at least as it applies to their podcast.
The writing and reporting were solid, but I missed the humor we often get from them in Crime Writers On... Granted, there's not a lot funny about the murder of Lizzie Marriot—the story almost made me feel grimy when I'd put it down. Still, a pretty good entry in the true crime genre.
The writing and reporting were solid, but I missed the humor we often get from them in Crime Writers On... Granted, there's not a lot funny about the murder of Lizzie Marriot—the story almost made me feel grimy when I'd put it down. Still, a pretty good entry in the true crime genre.
March 25, 2016
Very well written
Hard to put down.Well written-hooks you in from the first pages.Going to look for more books by this author.awesome!True Crime
Hard to put down.Well written-hooks you in from the first pages.Going to look for more books by this author.awesome!True Crime
September 26, 2018
I picked this up because I am a fan of Flynn and Lavoie's podcast Crime Writers On, and I wasn't disappointed. This is the true story of a couple of young adults in New Hampshire and the awful crime they committed.
Kat McDonough was 17, loved drama (as in acting), and, like many 17 year olds, was angry with her parents. This made her the perfect target for the much older Seth Mazzaglia. Seth lived in a rich fantasy world with BDSM tendancies, and in Kat he found someone he could mold and control.
The book tells the story of both Kat and Seth, their backgrounds, how they met, and how their relationship progressed until it climaxed in tragedy. It's an interesting story about the potentially dark world of BDSM (don't get me wrong, most people who practice BDSM don't act like Seth) and a young girl who felt trapped and got herself in way too deep.
I liked the book and the audiobook was a decent production. I do wish they had fast forwarded through their early relationship a bit, but I understand the need to show how Kat got pulled in so far and so quickly. This was a pretty good true crime read.
Kat McDonough was 17, loved drama (as in acting), and, like many 17 year olds, was angry with her parents. This made her the perfect target for the much older Seth Mazzaglia. Seth lived in a rich fantasy world with BDSM tendancies, and in Kat he found someone he could mold and control.
The book tells the story of both Kat and Seth, their backgrounds, how they met, and how their relationship progressed until it climaxed in tragedy. It's an interesting story about the potentially dark world of BDSM (don't get me wrong, most people who practice BDSM don't act like Seth) and a young girl who felt trapped and got herself in way too deep.
I liked the book and the audiobook was a decent production. I do wish they had fast forwarded through their early relationship a bit, but I understand the need to show how Kat got pulled in so far and so quickly. This was a pretty good true crime read.
June 1, 2022
The problem with Dark Heart is that we will never know what happened the night Lizzi Marriott was murdered. The story up to that point is one of the strangest that I have ever read. Dragons, spirits, fantasy role playing, bondage, torture, three way sex and we have an extended version of Penthouse Forum. Kat McDonough and Seth Mazzaglia are the pair of star crossed sadists and centers of attention who provided he said, she said accounts of Ms. Marriott's early demise. By the end of the book, I wished for both of them to be shipped to one of the dungeons of their fantasy world. A fair warning to those offended by graphic descriptions of S&M. Stay away from Dark Heart. Meanwhile, I was most mystified by the quirks of my fellow human beings. Overall, a highly entertaining read.
February 20, 2023
This is a well written true crime novel, however some of the subject matter is just really disturbing (trigger alert: sexual abuse). Cat herself was clearly abused by Seth Mazzaglia, which doesn't excuse what she did but gives it some explanation. I was pretty frustrated by Cat's parents. "Our high school aged daughter is dating someone in his 30's? OK, well let's use ice cream analogies to try to get her to dump him instead of reporting it to authorities." I mean, I know they were dealing with their own problems at the time but talk about denial and inadequate parenting... Seth himself is clearly a sociopath and, fortunately, he's rotting in prison. What happened to Lizzie and what her family has had to live through is really sad.
August 9, 2018
To be honest, I am only giving this a 4 star review because it seemed to take a very long time to get to the actual story. A lot of the back ground was told during the beginning which made it seem at times like it was a book about kinky sex. Around pages 50 through 100 everything started happening very quickly, and I could not put the book down. I had to know what happened when Lizzi got to the door of "the lair". The story was extremely disturbing and even though I feel personally that Kat was a victim of violence, I feel like she deserved a longer sentence for what she did. Seth is one disturbing/ disgusting individual and I will never forget this story.
January 23, 2019
Writing-3.5
Story-non-fiction
Enjoyment-that's too strong a word here...
I was interested in reading about this crime that happened very close to home that I hadn't ever heard about (the joys of being cut off from the local news circuit) so I picked this bad Larry up. The title is cheesy but makes sense once you get to know the player. This was a tough read because all the people in it are real and despicable (except Lizzi and her family). Well, really only the two main persons are fucked, I can't really blame their families for how they turned out. Anyway I'm rambling. It's a pretty good true crime and I recommend it if that's your jam!
Story-non-fiction
Enjoyment-that's too strong a word here...
I was interested in reading about this crime that happened very close to home that I hadn't ever heard about (the joys of being cut off from the local news circuit) so I picked this bad Larry up. The title is cheesy but makes sense once you get to know the player. This was a tough read because all the people in it are real and despicable (except Lizzi and her family). Well, really only the two main persons are fucked, I can't really blame their families for how they turned out. Anyway I'm rambling. It's a pretty good true crime and I recommend it if that's your jam!
March 27, 2019
A lurid and senseless murder
This is a very well researched and written true crime book. The death of young Lizzi was so hard to read about at times. Seth is simply just a monster and a loser who abused every woman he came in contact with. And Kat? I think she was only a child when she met Seth and was a battered woman in some ways. It's hard to believe New Hampshire has no conspiracy to commit murder penalties. This book is extremely graphic so make note of that before reading if you are easily offended by alternative sexual lifestyles. Great book. Very sorry for Lizzi's family.
This is a very well researched and written true crime book. The death of young Lizzi was so hard to read about at times. Seth is simply just a monster and a loser who abused every woman he came in contact with. And Kat? I think she was only a child when she met Seth and was a battered woman in some ways. It's hard to believe New Hampshire has no conspiracy to commit murder penalties. This book is extremely graphic so make note of that before reading if you are easily offended by alternative sexual lifestyles. Great book. Very sorry for Lizzi's family.
June 20, 2022
Not for the faint of heart, although if you pick up this book at all I doubt that you are squeamish. The title alone lets you know there is probably some graphic moments. Personally, I think the authors could have spent less time on the graphic sex and more time getting into the three main characters heads and what made them tick. The main perpetrators here is Seth Mazzaglia and his girlfriend Kat McDonough, who are into fantasy video games and like to role play. Along the way, the befriend a girl who accidently is killed during one of their sex role plays. The story is interesting, not so sure I agree with how the sentencing played out. It seemed like one of the perps got off pretty easy.
March 10, 2025
A harrowing and disturbing story.
The authors presented this extremely sad story by first honoring the memory of the victim. I think Seth was a sick sadistic & cruel individual. You see the crux of this story in other men that seduce & control young women in order to satisfy their libido & ego. Jim Jones, David Koresh, Polygamist seem to find the ones with low self esteem & troubled home life. I'm torn about my feelings for Kat, she was only 17 but seduced by a 29 year old man. Yet she should have known right from wrong. I couldn't put the book down kudos to the author's
The authors presented this extremely sad story by first honoring the memory of the victim. I think Seth was a sick sadistic & cruel individual. You see the crux of this story in other men that seduce & control young women in order to satisfy their libido & ego. Jim Jones, David Koresh, Polygamist seem to find the ones with low self esteem & troubled home life. I'm torn about my feelings for Kat, she was only 17 but seduced by a 29 year old man. Yet she should have known right from wrong. I couldn't put the book down kudos to the author's
October 11, 2017
As far as true crime books go, this is pretty good. The background story on the couple was interesting but the ending was just sad. How could Kat have to serve such little time for this crime is beyond comprehension. Would be interesting to see where her life is now, since she has been released from prison.
May 24, 2019
Craziness! I’ve been looking for a Kevin Flynn - Rebecca Lavoie book to read and chose this one. I know the “alternative lifestyle” if some of the subjects is a key point of the case but maaaaan I didn’t need so much of it. That said, I cared enough at the end to Google an update... which was as disturbing as the book itself.
September 19, 2018
Kevin Flynn is an exceptional true crime author. I stumbled onto one of his books a few weeks ago via an Amazon recommendation and I've since torn through everything he has written in the genre to date. I'll be eagerly awaiting his next publication, whenever it is released!
January 31, 2020
One of the best true crime novels I’ve ever read!
Loved the story telling and how much detail was put in.
This is the first time I read a true crime book where I knew nothing about the case.
This book is truly under rated!
Loved the story telling and how much detail was put in.
This is the first time I read a true crime book where I knew nothing about the case.
This book is truly under rated!
March 25, 2020
Overall, I enjoyed the book, but I think there were some massive holes. Obviously, the only people who know what really happened either cannot speak or won't be honest, but it feels as though the story was very much incomplete.
November 26, 2021
Never dull. A very complete book beginning with the way the killers met and began a relationship that turned to BDSM. In the end I am unsure if it was the male or the female partner that instigated the events leading to the murder.
October 16, 2025
This was a really captivating read. Such a horrible crime and the authors really brought characters to light. Some really cringy facts brought together exactly who the killer was as a person. The best book I’ve read in awhile
January 2, 2018
Sick details, but every young woman should read this so that they know what manipulation looks like.
April 17, 2018
I'M NOT A FAN OF THIS BOOK, ITS A BIT TOO GRAPHIC SEXUALLY FOR ME. HOWEVER, IT IS A GOOD STORY AND THERE'S A LOT TO LEARN ABOUT RELATIONSHIP VIOLENCE AND HOW A LOVER CAN BECOME A VICTIM.
July 27, 2019
I could not put this book down.
January 12, 2022
Super creepy. Lot of parallels between places and names for me that made it even worse. Almost surreal and definitely said “oh gross” and “holy sh!t” more than once.
September 21, 2016
I've been reading a few of these novels, especially liking the ones which take place in NH, since that's where I'm from.
This one was especially intriguing, since it's so current and I recall reading about the events as they unfolded in the news.
If you're not already familiar with the story, it's about a girl named Lizzi who is in college and doesn't come home one night. Fast forward and they discover her last known whereabouts was at the apartment of Kat and Seth. Initially they try to deny they had anything to do with it, but eventually the police are able to determine that she was either the victim either a sexual act or Seth's rage in being rejected. Only Kat and Seth know at this point. They dispose of the body, which has still not been found.
It wasn't what I expected in the book, there's a lot more about Kat than there is of Seth, but from reading the book, it sounds like that was the intent, so I get that. There's a lot of background on Kat, trying to explain what would cause her to become involved in the relationship.
It's pretty disturbing to realize that there are people out there who would behave this way.
This one was especially intriguing, since it's so current and I recall reading about the events as they unfolded in the news.
If you're not already familiar with the story, it's about a girl named Lizzi who is in college and doesn't come home one night. Fast forward and they discover her last known whereabouts was at the apartment of Kat and Seth. Initially they try to deny they had anything to do with it, but eventually the police are able to determine that she was either the victim either a sexual act or Seth's rage in being rejected. Only Kat and Seth know at this point. They dispose of the body, which has still not been found.
It wasn't what I expected in the book, there's a lot more about Kat than there is of Seth, but from reading the book, it sounds like that was the intent, so I get that. There's a lot of background on Kat, trying to explain what would cause her to become involved in the relationship.
It's pretty disturbing to realize that there are people out there who would behave this way.
March 3, 2016
Not like most crimes, or criminals
Very well written, heartbreaking, violent. I fail to understand what motivates people who commit such crimes in the name of pleasure and power. I'm not sure what to think of Kat McDonough. She fell prey to an older man...not the first girl to do so, or the last. But at some point she had to realize something was grievously wrong? Right? The tragedy is illuminated in the posters of Lizzi Marriott. When could this have been stopped? The only meagre consolation is the monster Seth Mazzaglia rotting in prison , and eventually cast into the pit of obscurity. Good job Rebecca Lavoie and Kevin Flynn on the research and writing of a difficult story.
Very well written, heartbreaking, violent. I fail to understand what motivates people who commit such crimes in the name of pleasure and power. I'm not sure what to think of Kat McDonough. She fell prey to an older man...not the first girl to do so, or the last. But at some point she had to realize something was grievously wrong? Right? The tragedy is illuminated in the posters of Lizzi Marriott. When could this have been stopped? The only meagre consolation is the monster Seth Mazzaglia rotting in prison , and eventually cast into the pit of obscurity. Good job Rebecca Lavoie and Kevin Flynn on the research and writing of a difficult story.
Displaying 1 - 30 of 44 reviews