This is terrible and I feel for you. I know you probably don't care and you wish you hadn't interfered on hindsight, but you did what you felt was right at the time and that is both brave and commendable. The guy is obviously a douche bag and an asshole but it's possible you did save his life, though I know that's probably not consolation for you right now.
Try your hardest not to let this incident permanently influence your perspective on other individuals but above all take care of yourself
Actually, I would do it all again because had I not interfered, the attacker either would have killed the victim, or no one would have gotten involved, enforcing the status quo until the victim was eventually murdered by his partner. As angry as I am at the victim right now, had I decided this was not my business and abandoned him there on the floor, I could not live with myself ever again. But because I did do something, my conscience is clean.
It's the only part of me that doesn't hurt right now.

I can't say that for anyone else involved in this. While I was at my apartment building on Tuesday, my neighbors scurried past me with their heads down and wouldn't look me in the eye. I think they know what they failed to do, and they're going to have to live with themselves over this for the rest of their lives.
Me? I can live with myself.