The story here seems to be that the guy in the black vest pulled a knife on the aggressor, who seems to have a few friends with him, which probably explains his large attitude.
Anyway, we see no knife in the video. He allegedly pulled it before the video started rolling, then got cornered. But all I see here is a guy that clearly doesn’t want to fight, getting knocked out by a guy who clearly gets off on it.
Obviously, we can’t say for sure whether this is justified or not. If there was a knife pulled, then maybe vest-man deserved what he got, but if not, then this is pretty out of order. It’s one of those that you really can’t judge. But what I can say is that elbow-thrower is a grade A knobhead.