Well, unless your State has something like Florida's Stand Your Ground law, you must bug out, unless your life is threatened. With the BG's back to you it would be hard to convince the DA.
I believe that states requiring retreating before use of lethal force are in the minority. I don't recall there being more than a couple that required this, but I may be in error.
There are many reasons to shoot a bad guy in the back, and not all pertain to your life being threatened. While you might not be in direct danger from a guy with his back to you, there may be others in front of the bad guy whose lives are being threatened.
In some places, lethal force is justified to stop various non-life threatening or no longer life-threatening jobs. For example, the owner of a store may shoot robber making away with all of the store's money. Besides, whether or not you may be in danger is usually superceded by whether or not you perceived yourself still being in danger.
Given there are no laws in the US that preclude back shots, shooting a bad guy in the back isn't going to be that significant.
You know, if the bad guy is so stupid to make the tactical blunder of turning his back on your, shooting the bad guy in the back potentially represents the absolutely best time for the task. If the bad guy can't see your actions, then he isn't likely to try to be defending your actions.
Clint Smith once said it and I agree. "If you're ever in a fair fight, your tactics suck." Amen, brother Clint.
There is nothing unfair about shooting a bad guy in the back when it was his decision to present his back to you.
In the Tacoma mall shooting, the bad guy was bent on murder and mayhem.
The citizen with the gun, the good guy, wasn't bent on murder and mayhem, and as a result, hesitated and issued a verbal challenge when he should have fired instead.
Note the news story where the good guy says he just couldn't shoot the kid in the head.
There are several reasons why McKown failed to shoot the bad guy in the Tacoma Mall. First, he didn't know the law and so was afraid of getting arrested for brandishing. He did not want to get in the way of police if they were handling the situation, only there were no police there at that time and defending your life or the lives of others in such a situation isn't going to be getting in the way of the police in an active shooter situation.
Of course he claimed that he would have to shoot the bad guy in the head and it is hard to shoot someone in the head, as if he had tried.
The primary reason McKown did not shoot the bad guy because he didn't even have his gun drawn.
Given that McKown claimed to carry a gun to protect others, that wasn't going to happen because of his fear of brandishing.
