I have rule that I've never tested. It says don't use your gun to defend anyone but yourself, your family or your friends. I'm not a cop. I'm not paid or trained to defend the public in general.
I would also be reluctant to use a gun when the other guy doesn't have one. That's because I know someone who tried to defend a woman, managed to divert attention to himself and was beaten badly enough that it was tough to recgonize his face as something human. During this last beating, he drew a gun and shot the guy in the foot. He was charged, tried and acquitted for attempted murder and assault with a deadly weapon (or something like that). He lost a civil suit for damages. Two or three jurors were asked why and they said he had a gun and the other guy didn't.
Another rule I have - also untested - is don't get involved in a physical altercation if you are armed. It risks getting to the point where you may have to use the weapon just to prevent yourself from losing it.
So being armed makes me timid to some extent. If I wasn't armed in this situation, I'd probably wade in. Not with the idea of winning (I'm only 5' 9" and about 175 pounds) but with inspiring the rest of the crowd to intervene.