The punch might have been a bit justified, because the guy picking the fight was being a bully. However, a different tact might have been more prudent. You can't just sucker punch someone because you don't like the way they are acting.
Consider for a moment that the guy fell and hit his head, which put him into a coma. This video would have become evidence in an assault investigation. Consider that he died while in the coma. Then it becomes either a manslaughter, or homicide investigation.
While the guy was an idiot and deserved to look like a fool, it was the guy who punched him who could have become the bigger fool, and thrown his life away because he tried to play the hero.
Just my perspective.