Jeff, honestly.
Honestly, all actions that one takes will be judged by the "reasonable person" standard. Holding oneself to the "try to think of all possible situations and ensure 100% success given each one" standard is simply not possible for either cops or civilians, and ensures 100% inaction.
And I still haven't seen a sourced example of a CCW holder shooting a bad guy and killing a civilian accidentally. Doesn't mean it hasn't happened, but it's pretty rare if it does.
I also haven't seen a sourced example of a CCW holder being shot by arriving cops due to mistaken identity. Doesn't mean it hasn't happened, but it's pretty rare if it does.
One can prognosticate infinite possibilities of what has at least a tiny chance of going wrong. The reasonable person takes reasonable precautions and makes reasonable assumptions.
In the OP (you know, the one we lost track of), we know that the robber has (a) a real gun (b) intention to use it and (c) is a criminal. We make reasonable assumptions about him and his behavior, and if we deem it reasonable, we shoot. If not, we do not. Simple. And if he's firing the gun into the ceiling, I'd say it's pretty dern reasonable to shoot him at that point.
(and for the purposes of this discussion, I probably wouldn't definitely shoot the guy unless at least one other person is executed. At that point, it doesn't matter if he has accomplices, if he's wearing armor, if the police are on their way or not, or whatever other scenario one can create. All I know then is that me and everyone else in that room from that point forward is dead already, and I might as well do something.)