I appreciate your view point. I think MOST people would claim that if they were armed and close by and witnessed a BG blasting away innocent people (eliminating witnesses) they would not try to sneak away but try to save these innocent people.
If you are really concerned about this kind of situation, and I realize you're mainly trying to think ahead, you should consider talking to local law enforcement, talk to a lawyer (if you are considering being a bit of a vigilante, you should have one on speed dial), get an umbrella policy addendum for your insurance (it's cheap), and talk to some store managers.
I think you'll find that the cops will tell you to leave the policing to them, the manager will tell you to let the insurance cover it, and the lawyer will tell you how every bullet you fire has legal consequences. Learn as much as you can about what the consequences are of stepping up. Sometimes you can get into trouble when the perps relatives sue you in civil court.
In reality, I think you can go a lifetime without ever having to make a decision to take a life to save lives especially if you heed the 'avoid stupid places, stupid people and doing stupid things' rule.
The truth is, a civilian who carries should always apply the 'in the Gravest Extreme' rule and never discharge their firearm unless there is absolutely no alternative. Always seek to evade and escape, because in a gunfight you never know what is going to happen. You could hit an innocent, you could cause the perp to jerk the trigger and shoot the cashier, you could be shot by mistake by a LEO who sees you with your gun drawn. Your duty is to go home to your loved ones. Make that your first priority and you shouldn't go wrong.