Well, I am disappointed by all of the "it's not my problem" attitudes in reply to this scenario. I think that the course taken has to be the one that is estimated to have the greatest probability of the innocent remaining physically unharmed. When someone points a gun at a person who is not threatening anyone, he has made it clear that he is willing to take innocent life. Robberies often turn into shootings. I do not know the statistical incidence of this; perhaps somebody out there has some data. I agree that a scan all around for an accomplice is important. If there is another present who might be an accomplice, then that raises the odds of escalation if a gun was pulled where the accomplice could see it. However, I do not think that an accomplice would rush in and make himself known if he saw the first man go down with a bullet to the brain from an unseen shooter. He would run like a rabbit. An attempted draw where an accomplice could see it would be dangerous, however. Now I wish to move past the matter of an accomplice.
I suppose there would be the possibility of the bad guy looking like the sort who is no threat, but that does not seem likely. Armed robbers are generally repeat offenders who are willing to kill, and he has demonstrated that he is willing to kill when he pointed the gun at the clerk. A way to end that threat would be to put a bullet into the center of his head, interrupting the CNS processing necessary to send a signal to the muscle that flexes his trigger finger. If there is nobody near the line of fire beyond him, which I have said there is not, then this should not be a risky shot. The clerk is not in line with him, which I tried to make clear. If necessary and possible, take a few steps to the side to align properly at a different angle. If a line of fire cannot be obtained which is free of the innocent, then the gun should remain holstered. Focus on the front sight, and stop the threat. Any warning such as "drop the gun or I'll shoot" raises the probability of the innocent getting hurt. I believe that the highest probability of the innocent remaining physically unharmed, in the scenario I described, is to silently draw the gun, assume a proper two handed grip, align the sights with the middle of his head, focus on the front sight, and squeeze one off without warning.