What should I do if I think someone I know is a potential murderer?

With the mass shootings that gained great news coverage, I was intrigued to read upon the visible psychological warning signs of such personalities, and it disturbed me to remember that I once lived with a person who casted off these signs. He was a roommate who started off extremely friendly, but mentally deteriorated rapidly. Whereas he started warm, he was soon walking around the house with a paranoia-stricken face, and threatened me with a stiletto knife in asking whether I had been snooping in his room. (I hadn't, and he had no reason to think I was.) Through his own admission, I learned he regularly likes to bare-fist fight people on the streets in illegal fighting contests, and, by searching an address from his mail, learned he served time for a violent assault on a landlord, where he attacked a man with a tire chain and struck his wife. Altogether the picture is of a man struggling with severe mental issues who feeds them with his fighting habits, and has factually caved into violent urges. There's the potential that someday he could literally commit murder. I no longer deal with this person, but wonder what I could have done. How should I respond when I think someone has the capability to literally commit murder, as backed up by his violent crime history, his fighting habit, and his visible mental deterioration?