I Hunt Killers is certainly not a typical YA novel, in fact it doesn't feel very YA at all. It is difficult to feel "young" when the topic is serial killers and the main character is the seventeen year old son of a well-known prolific serial killer that groomed his son to be his "replacement". Then there are the gruesome details, deaths and psychological twistedness/thriller nature of it all. In fact I would say it felt more like a Michael Connelly novel than anything else, just perhaps a little less involved.
Jasper ("Jazz") is a character who is twisted but sweet at the same time, it makes it confusing at times whether or not you should actually like him....but ultimately you do. He continues to question whether he is evil like his father, a realistic situation when you consider he was involved in killings and helped keep track of killing treasures etc. for his father while growing up. He is continually haunted by his father's voice in his head and nightmares. He honestly doesn't know if he is "good" or "evil" and questions every action he makes as potentially done for evil reasons. Jazz was well-drawn and I really rooted for the kid, although had I been his girlfriend in the book (Connie), I don't know if I would be as certain of his good nature.
The secondary characters of Howie (best friend), Connie, the sheriff (who actually is on Jazz's side) and his grandmother were well written as well. I liked Howie, he was somewhat of a comic relief. Connie, the girlfriend, was strong and good for Jazz although I could have done without a girlfriend theme altogether, I found it somewhat distracting. (Although it can be argued that Connie kept Jazz on the straight and narrow.) The sheriff was good, although I wonder how realistic he was in regards to the whole situation. The grandmother--ugh, she was well-drawn to the point that I could not stand her. These types exist and holy crap they annoy me, so every time "grandma" started ranting I wanted to tune out.
The mystery was well-written, although not as thorough as an adult mystery. Ultimately there were only a select number of suspects early on, although it could have been any one of them even towards the end (you never know, the author can throw whatever they want at you so claiming to "know" who was the killer isn't quite accurate).
Overall I rather enjoyed I Hunt Killers, it surprised me by the darkness of it for a YA. The mystery was good, the characters were well-written and it was overall entertaining. I look forward to reading further books in the series.