If you truly consider him armed and dangerous to others, I would report to authorities. On the other hand if he felt this was the "last stand" it could become deadly. Although the same could be said if you don't talk to somebody. Does he have family you can speak to? That might be the safer initial route.
He lives with his mom, but she's a nut too. Not like him, but the crazy Jesus type who let her cat drag itself after it had a stroke and was paralyzed below the waste because "god's will". My problem is if I call the authorities, what can I say? Our only conversation during this was him telling me to fuck off after I asked him to talk to someone professionally. "Yea, my friend told me my college room mate has boarded the crazy train and is armed."
My uncle is the retired dispatcher for the police. Maybe I should call him.