What an interesting story. 
 I'm sure the comparison has been made, but this character kept reminding me of Dexter. When he was explaining his lack of empathy, I remembered that scene after Rita died and Dexter was with her kids and parents, and he said to them, "I'm sorry for your loss." It was the most hilarious (and saddest) part ever because it was such the wrong thing to say, but he was just copying the funeral director. He was trying really hard to be normal. Dexter and John are also similar for the fact that they're good people struggling with bad urges, of course, and they both find relief in tracking other killers. (At least, that's where we have John in this volume of the series.) 
 But I digress, because really, this story was nothing like a Dexter episode. It was sort of a mixture between Dexter (the "monster" aspect) and the main character in The Curious Incident of the Dog in the Night-Time (the lack of empathy and understanding people in a teenager), with a paranormal element and some mortuary action added for fun. John was alternatively hugely likeable, kind of weird, and creepy as hell. It works for the story, and unless you're the kind of person who needs to love everyone in the story, it will only heighten the believability of the situation.