I Am Not A Serial Killer by Dan Wells

The book follows John Wayne Cleaver, a teenage sociopath who lives in a small town and is obsessed with serial killers. Despite his dark thoughts and tendencies, John adheres to a strict set of rules to prevent himself from becoming a killer. However, when a series of gruesome murders occur in his town, John decides to use his unique perspective to track down the killer. As he delves deeper into the investigation, he discovers a supernatural element to the murders, forcing him to confront his own inner demons and the nature of evil itself.