Should I stick by my violent friend?

well hes not really my freind anymore, he went over to a kid in our school, half his size and randomly punched him round the face and this little boy was crying, I said you had no right to do that then slapped him… he went mental at me holding his fist in my face and screaming at me and pushing me… he had to be stopped… I’ve been freinds with him for a while, and he has issues.. but I never knew he would do this to an innocent guy… and also he told me he wud never think about hurting me when he almost punched me… help me please should I try stick by him or should I not be freinds with him, even thou hes mentaly destroying me…

p.s I slapped him for his own good… but it backfired (it wasnt a gesture to harm him)