For me, there certain criteria I want to have met before I get actively involved.
1. I must have a clear understanding of what is happening.
2. I must know who the "players" are.
3. I MUST be able to POSITIVELY ID who is the innocent and who is the aggressor.
(Knowing that those roles are not always present during a conflict.)
If I can't figure out what's going on, why, and who the BG is, I'm probably not going to be engaging anyone.
Now, if a guy walks into the church with a shotgun and starts blasting every 3rd person in the aisles, that's clear enough for me! I'll try to stop him, if I have a good opportunity. If not, I'll try to get as many people out of the line of fire as I can. (Assuming I don't just stand there with a dumb look on my face!
I won't fault people for simply defending "me and mine." That is a reasonable and safer course of action. I would simply feel awful if I knew I could have helped the victims but didn't (which may or may not be the case, depending on incident specifics).