Another way to say "don't play fair, play to win because your life depends on it."I agree and have no problem with this scenario of shooting the perp in the back.
For those that have some visceral or nagging issue with "shooting him in the back", look at it this way. He still had the gun. He was still in the restaurant. He was still waving it around. He was still pointing it at patrons. In short, he was still a threat.
For those with an issue with it, think about why you have an issue with it? Is it some old Western "John Wayne" ethos of "only a dirty rotten' varmint" would shoot a man in the back instead of facing him? That's Hollywood showdown gun fighting claptrap. Are you analogizing this to an "assassin" who might slip in behind his target unnoticed and pop him in the back of the head? or something along those lines?
This is not any on those.
Is it some sense of it not seeming fair?
I would encourage you to watch some of the ASP channel videos on Youtube that analyze hundreds, if not thousands, of these interactions. If the bad guy has a gun, you have to "wait your turn". The 1.5 second draw to first shot time is not to "outdraw" him while he has the drop on you... it's to take advantage of a short window of time when he might be distracted and focused elsewhere... the mother of all distractions is if he turns his back to you.