Find out if your boyfriend ever harmed another person. If he did, you must move out immediately in order to protect yourself.
People who commit serious violence against other people often have a history of either violence against animals, or lesser violence against grown-ups or children, or of arson, etc.
It's also possible that your boyfriend is suffering from an Obsessive-Compulsive Disorder. If that’s the case, he's struggling with obsessive thoughts. His subjective experience of stress indicates that he has an anxiety disorder rather than a ruthless psychopathic personality.
If he's suffering from OCD
he should seek a psychiatrist to get proper treatment, for example, Prozac/Fontex (Fluoxetine) 60 mg/24 hours. Ask him to follow the psychiatrist’s directions in detail.
Addition to this answer by one of our users
I have worked for almost 9 years as public safety in the 911 arena and my advice is to move out until your b/f can find out if he has or does not have OCD. It's a little too late after the fact. My experiences have been if they say that they're gonna do it or imagine doing it, then it's not too far ahead. I'd consider it a big, red flag.