One of my first weapons classes gave the following scenario:
You're standing in your yard and you see two men walk up and shoot a third man. They then turn and walk away.
What are you legally allowed to do and can you shoot them?
The answer depends on what they do next.
If they continue to walk away, you can not shoot them and if you persue them and they feel threatened, then they have the leagal right to defend themselves.
If they turn and make a motion that makes you believe they are going to shoot the 3rd person again, then yes you can legally shoot at them.
There is a very fine line between when someone is the aggressor and when the "victim" might become the aggressor. The situation works in the reverse also; because an aggressor can become the victim, depending on the behavior of all parties involved.
If you're in a bar and someone punches you in the face, you have the right to defend yourself; but if you knock them down, you can't legally stomp a mudhole in their ass.