Ellie is a martyr who cannot say no and takes on tasks that she has no business being part of. The police are lazy and ineffective, arresting the easiest person with no evidence, and somehow it's up to Ellie to endanger herself for a man who should be capable of handling things on his own. Neil is an ADULT. He is not her relative or her love interest, or even someone that the reader cares about. Yet she spends waaaaay too much time thinking about how to get him out of trouble with the police. When Ellie stumbles across the answer, there is a predictable confrontation with the murderer in which Ellie's life is predictably threatened, and then everything is swiftly cleared up (but only because the author has been giving you updates on what the murderer is doing and thinking, and not because there are any clues).
Also, I don't understand how things with Thomas went so quickly off the rails, and then is put back on track by him showing her his office and asking her advice. What?