Speaking for myself.....
I've had LE training and background, along with other weapon and training experience.....and have been carrying for 20 years, largely on my CCW permit.
I know I couldn't dream up every possible scenario which might occur, but I've spent considerable time over the years thinking about my correct response based on different circumstances.
Even as a police officer, I was trained that if off duty but armed, and one found oneself in the midst of an armed robbery with civilians present, the best initial response is to be a good witness. Pulling a gun in that situation, lacking aggravating factors, is only likely to start a gun fight, and kill or injure innocent victims or myself.
So, first, be a good witness, but if it appears that another persons life, or my life, is in imminent danger, then one should be prepared to do what you need to do in that case.
The first thing to do is to pay attention, have my gun in my hand discreetly, or at least my hand on it, and if I could move without drawing the BG's attention, then I'd want to find cover where I can also see what's going on.
For me, the line has always been, and is, that if the BG fires his weapon, whether he hits anyone or not, then all bets are off, and I'm going to act. If the BG is threatening GBH or death, then I'm not likely to intervene.
In the grocery store case in Milwaukee, I give the GG points for being discreet, and gradually easing himself into a position where he had a line of fire away from innocents, and he just didn't announce and jump right into the middle of the events transpiring around the cashiers area.
However, I don't believe I would have fired in this case, but certainly would have had my pistol out, behind my leg or otherwise out of view, and maneuvering as the GG did in this case, but also keeping cover in mind....which he did not do. The BG could easily have started shooting at the GG or other innocents, and who knows what the result of that would be....possibly very very bad.
The BG was leaving, and I believe I would have let him go....
Just my thoughts....hope and trust it never happens.....
George