I DON'T WANT TO KILL YOU by Dan Wells

Sixteen-year-old John Wayne Cleaver has always known he's different, but not because he only has one friend (and doesn't much like him) and not because he regularly helps out in his mother's mortuary. He's different because he recognizes the classic signs of an incipient serial killer in his own personality, and he's created a rigid set of rules to follow to keep his darker nature, the one he calls Mr Monster in check.
But John discovers it's the personality traits he so fears that put him in...