Posted by jibberjabber: I wouldn't fire the first shot. The BG might just leave if he gets what he wants. But if the BG started shooting at people, I'd try to intervene. One thing though, I wouldn't want to shoot any innocent people, so I'd need to shoot from a distance where I was confident of not missing. So really, really close then.
You might want to consider one other thing: if one or more perps start ordering everyone into a back room, there's a pretty high likelihood that step two will be to get everyone onto the floor, and that step three will be to start killing people.
At some point in that sequence you will lose your ability to act.
You may be well advised to seize the opportunity while you can, if it is pretty clear that the situation is really going downhill.