Fuck yes it was justified. Anyone who says otherwise is a gibbering idiot or so ridiculously biased that their opinion is worthless. He hit an opponent when he was down and in a vulnerable position and clearly thought about it before hand. It wasn't like he was in the midst of punching when Dirrell went down, he saw he was down, thought about it, and them he hit him.

I'm a huge Abraham fan, or at least I was, but that was a chicken-shit move and he acted like a classless asshole afterwards.