Originally posted by GScholz 
Just shooting someone because they're holding or firing a gun isn't enough. The courts will later decide if you were justified in your actions. If I see a man about to kill an unarmed and obviously non-threatening man, I will try and stop it. If I am armed I might even kill the assailant to save the other man. If I killed a man just because he held a gun, or if I didn't see what he was shooting at, I would most likely be charged with second-degree murder. 
 I guess you still don't see the huge gapin' flaw in that.  So I am walking down the street, and I see you about to fire a gun upon a guy about to fire a gun.  Under you logic, and I do recall you saying this was 100% legal where you are from AND ones civic duty (you seem to be backpedaling a bit), I should shoot you to keep you from shooting them.  That's what you said,   I would be defending the guy you are about to kill, and under your premise it would be perfectly ok to kill you.
Mull that one over a bit please and see just how silly it really is.  The whole premise of taking a life, to prevent the taking of a life is not only nuts, but is horribly flawed logic.  I trust you see the hypocrisy in your analogy.