In most States the person who comes to the defense of another steps into the shoes of the person to whose defense he came. That would mean that if the person you defended would have, under the circumstances, been justified in using lethal force to defend himself, you would be justified in using lethal force to defend him.
But here's the rub. Do you really know enough about what has happened to know if the person you plan to defend would have been legally justified in defending himself? For example, would you know enough about the situation to know that the person you're about to defend was not actually the original aggressor? What if the guy you're about to defend actually started the fight?
Originally Posted by DogoDon
...Of course, there may be situations where it's clear that the victim is in need of aid (e.g., a man mercilessly attacking a woman)...
And you still need to be careful. I seem to recall an incident about a year ago in which someone came upon a man who appeared to be attacking a woman. The knight-in-shining-armor shot the man. It turned out that the man was an undercover cop and the woman was a hooker who was violently resisting arrest. The k-in-s-a went to jail.