You don't have to allow someone to attack you...but you don't have to kill them either. Basically, because that's messed up. By all means, if conceal and carry is permitted, then pull out your gun to stop the attack, chances are in a fists vs. gun fight, the attacker will back off without shots fired, and if not, then do what you gotta do. But taking a surprise punch, as in the OP, and then shooting the attacker after the attack has concluded is no longer self defense (if that's what happened, I watched the video, which didn't show the attack, from what I can tell, and I couldn't put the volume up). If that's not the case in Texas, well...my condolences, but you certainly have no claim to being considered civilized if revenge killing is permitted by law, which the use of a gun after the conclusion of the attack would definitely be.