What are the generally accepted Good Samaritan "rules" of CCW carry?

I've been carrying awhile and I think about this often and I don't really know the answer. Of course I know the "rules" of self defense use of a personal firearm. I know that if I feel my life is imminently threatened I can defend myself.But what about another person like a bystander? If I see someone getting attacked is it legal to draw? Do I have to wait until they threaten me?Just for the sake of argument use the classic example of walking down an empty street at night, passing an alley and seeing a woman being attacked..... via /r/CCW http://bit.ly/2Y0KiYa