First off, I doubt I'll be in line behind a robber. The first thing a robber will do is get in the back of the line, or come when it's less crowded. The second thing he will do is clear the immediate area around him.
However, if I AM behind him, I will simply put my gun to his skull and either allow him to drop his weapon or take his life as he turns to try to take mine. I have no problem shooting someone in the back of the head if he is turning to shoot me.
I seriously doubt that scenerio will ever play out though, for the reasons I've stated. When the BG robs at gunpoint, he is employing lethal force to accomplish the robbery. I believe the shooting of this BG would meet the criteria for intervention with lethal force in my state. I would not gamble the bank teller's life on the BG's goodwill. That bank teller could be someone's mother or father. If I felt the BG was a threat, and I could take him out with no others being hurt, I would do so. I can also understand why others might not.