This is where I come down on this.

On the one hand, there was no longer any threat to legally justify self defense.

But, despite a number of twitter users saying "well, what if there was a reason for the punch?" (including one nutjob saying "well, what if the black shirt guy raped the white shirt guy's daughter?!"), coming up behind someone and bashing him in the head is not justifiable either. And we all know there is minimal chance that the cops would have gone after the puncher (unless they could frame it as a racial thing)