re: Would you break up a physical fight between 2 strangers?

In the large majority of situations - no. Let them do minor/modest damage to one another and deal with it. I'm not leaving kids without a dad because I tried to intervene in a dispute I didn't know anything about to save shitty people from themselves.

On the other hand, if it's an obvious example of injustice of some sort, someone harming someone weaker without justification, then I think the stakes get higher (probably a better chance you're hurt/killed by someone like that) but the ethics become more complex. A world where people stand by and don't intervene in such situations isn't going anywhere good, and that has to be weighed against your personal health, your family, etc.