MC has a sadism kink but not really the sexy kind. Like, the "punches someone in the face during sex and fantasizes about cutting his partner" kind. He recognizes it's a real problem and sort of isolates himself away, fully expecting to live his life alone until a long-time friend confesses. They date but he ends it, fearing he'll hurt/scare away his friend. Some things happen that are a bit too close to some of the more violent/kinda terrifying fantasies he's been having when his friend ends up stabbed and huzzah, he's able to get off without imagining horrible stuff. Therapy would have been a good choice too.
Deen My Love