There's a few questions I have in this issue. Assuming it was in a State in which you are actually allowed to defend yourself, would it be legal to use lethal force against a gang of unarmed assailants? My gut says yes, but I still wonder in this messed up world we live in...
I would almost wager my house on the guess that if this person were to have pulled a gun, that the attackers would have left the scene as quick as possible. They were a group of teenagers, and I don't know many teens that would have taken a bullet to beat up someone else. If it came down to one of them having more testicular fortitude than the rest, putting him down would have certainly caused the rest to flee.
Also, I am at work and therefor have no audio, but it appears that he was pleading with them to stop. I have to say, that had I been unarmed and in the same situation, knowing I am probably going to get hurt no matter what, that I would have thrown the beating of a lifetime to as many of those thugs as I could.