stranger A (victim) is screaming bloody murder with stranger B (aggressor) on top of him with a gun. Victim is facing up with aggressor stradling him. Victim is screaming "stop it stop it, you're hurting me, leave me alone!" You yell at aggressor to leave victim alone, victim yells "help me, help me, he's going to kill me, I can't beathe!" Agressorr tells you to back off, it's none of your business, go away. You draw your weapon, point it at aggressor, tell aggressor to get off victim. Aggressor points weapon at you and you pull the trigger shooting the aggressor in the face, killing him instantly.

You're the hero right? You just saved victim? Not so fast my friend. Aggressor was an undercover police officer. Victim was a hard core criminal who was in middle of being apprehended. You just shot and killed a cop.

I just met your criteria, but it was still the wrong thing to do. I'm not saying your criteria is a bad thing, but I am suggesting there may be more to think about.

EDIT: My example was meant to illustrate that you never really know what a situation is just by what you see on the surface.