I've heard several times on this forum that if you don't think you personally are going to engage (weapon or otherwise) a threat, then you should stay back and "be a good witness."
I'd like to get everyone's take on this, but I see a couple of issues:
1. If I'm not engaging, that means I probably don't think its an actual threat to any innocent parties.
2. Even if it were an actual threat and I decide not to engage, hanging around to be a "good witness" could potentially make me and mine "collateral damage."
Obviously, each scenario would be different, but I'm leaning towards either engaging or leaving and calling 911 from safety.