I really enjoyed this whodunnit. I did not see the ending coming and it kept things interesting. 
Some of the "origin stories" were a bit harrowing, and it's not easy going in that respect, however, as we are trying to understand the motives of a serial killer it makes sense contextually. These messed up childhoods absolutely might create a serial killer. 
Kept me guessing right to the end and was actually quite gripping. 
Found this to be a slow start but really picked up in the third act and personally I could have done with a bit more pace, potentially this is an audio-book issue rather than a book issue. I liked it overall but I don't think I would go back to it.