TL:DR
Syntax/ sentence structure errors, lack of clarity, and also not as analytical as I had hoped. It gives the basics of each case without going in depth. Better to use this as a foundation and research online or through other informative books. Some grammatical errors and rushed to be written and done. More like an Index.
In specifics:
I expected to read an deeper analysis of each case but, instead, got an encyclopedia on each killer. There were some instances (I apologize for not having an example), where sentences were poorly structured, leaving the reader with a lack of clarity. In other words, it was hard to figure out who did what when there were several names mentioned at once, not clarifying which "he" did what.
There also are instances where the author mentions another serial killer at the last paragraph of an article. It confused me several times and made me double-back to see if I confused the name of the killer. Most of the time, it seemed random and placed only for the sake of taking up more pages. The author would connect two killers with one similarity and throw in a brief description of the other. Afterwards, I'd expect to see the mentioned name as the next case, thinking it was done as a lead in to the next killer--- but no.
Besides syntax, there were repeats of words and phrases every now and then. For this, I do have examples; there are two sentences that begin with "In fact" while only being paragraphs apart (Cimino 225, 227). The other instance can be found in the Allitt case where the phrase, "Turn for the worse," is said twice with being only paragraphs apart, as well (Cimino 251, 253). This may seem like nit-picking (and it basically is), but, as a reader, it causes disruption while trying to absorb the information when hearing a smidge of an echo.
Besides minor repeats, there was also lack of care given to the description of the cases. Like others have already mentioned, if you wanted to know more of the cases, might want to do your own research. This book can only serve as a surface level introduction of giving the basic per case (which isn't necessarily a bad thing). It gives the gist without going into exact details and interviews.
Lastly, since the beginning of the book had an introduction, I assumed there would be one at the end, too; there wasn't. It ended abruptly and without giving any insight or closure (though, I do understand why the author might want to hurry up this book. I wouldn't want to keep writing about these horrendous, real-life murderers, either).