I dunno if that is justifiable. If he hit the guy once or twice and the guy fell over and hit his head and died, then that may be justifiable. But if he hit the guy and the guy fell over, or even started to run away, at that point the attack and the danger to the victim has legally been stopped. Typically the "defender" has an obligation to stop at that point. If the father chased him down and/or continued to hit him after he was down and no longer a threat, then the father has become the aggressor in the eyes of the law. If that is what happened, then he could be charged criminally with some form of manslaughter or something. Just to be clear, I'm not condemning the father, I'm not defending the attacker, I'm not saying I don't understand the father's reaction nor how any father or parent might react to a similar situation. I'm just saying that depending on how the events actually played out, he very well could be prosecuted.