The complex issue is that you need to be dang sure you are seeing what you think you are seeing and don't drop an undercover cop.
In the reception line later that week, one of your friends gives you a "man hug" and says:
"Sorry about your wife (mother, sister) buddy. But you sure got that Bad Guy... Good for you, you did the right thing, don't beat yourself up over it."
I'm thinking that if you go through all the "what ifs" in your mind, whatever was going to take place will have already taken place while you stood there frozen by paralysis from analysis.
You get the best training and information you can beforehand, you train, and hopefully when the time comes, you will react in the proper manner. But, if you do react, it will probably be a reaction. There won't be a lot of thought process taking place at the time.
Is he holding the teller at gunpoint? If so, you are acting to protect her life, which is in immediate danger. Legit shoot.
Do your analysis here... Do your training for these odd situations there.
And, prevail without mishap (or at the least with a lesser chance of mishap) IRL, if you're lucky.
Annoys the hell out of other folks at the range though.
I don't believe that shooting someone in the back to stop a threat makes any difference in the eyes of the law. A person with a gun pointed at another individual with the intent to do harm = good shoot. If the LEO's, DA etc determine it was a justified shooting, the location of the entrance wound will be irrelevant. After all this is not a duel over honor, there is no need to try to make a gunfight for your life "fair". Waiting for the BG to actually point his gun at you so you are justified to shoot is pure silliness. I completely respect those that would choose to remain hidden or for whatever reason choose not to engage but it should not be out of fear of shooting someone in the back.
Side note: As I am sure you are all aware. The stigma of shooting someone in the back came mostly from movies and stories of folks in the "Old West" who would shoot someone days after a dispute when they saw them walking down the street and blasted them out of the blue in revenge.
As for your brain surgeon story above, we all have active and creative imaginations; we can dream up anything that is possible. OK? Do you think it would be a better choice to try to knock the attacker off center before shooting him? What if he gets a shot off as you slam him?
Yup, you could mind your own business, get a good description, let him out the door--- but oops, he shoots the clerk while you are being a good witness.
Look, anything and everything that we all can imagine happening will sometimes happen. Its a game of odds.