He died from smashing his head on the floor. Had they been on a beach the victim would likely have been fine. Not saying people shouldn't take violence even from unarmed people seriously. Defend yourselves! Just saying this was a freak situation, people are not so fragile that a punch in the jaw is likely to kill them as it seems some on this thread are saying.
Bingo! At least someone got it right on this thread. In all likelihood, he did not intend to kill. However, he is responsible for the result of his actions (head hitting floor and victim dying). Manslaughter.