BBI shares a robbery story in his class. It's actually one of the scenarios we did. The perp was holding good guy A at gun point. Good guy B, who was unknown to the perp, put a round in the back of his head from point blank range. Clean shoot.
That's the kind of scenario I was thinking of. I'm not about to yell drop it, etc. If someone in that situation needs shooting, they are going to get shot, I'm not about to give them any warning, etc and have no requirement to do so as a non LEO, etc.