Just to throw my hat into the ring...I think some of you are absolutely insane. I don't care if the law is with you, I would never want to be in the same room as some of you, especially with Amused.
I agree with JulesMaximus, that you have to look at this ****** logically. My personal opinion is that you should only shoot, especially to kill, if you have explicit first hand knowledge of whats going on.
1) Someone pulls a gun on you and threatens you. By all means, shoot the bastard.
2) Someone starts arguing with your friend while the two of you are next to each other, suddenly the guy pulls a gun on your friend and threatens him, but doesn't point at you. By all means, shoot the bastard.
3) You're walking down the street, and you hear a scream. You turn the corner and see one man on the ground, and another on top point a gun at him. STOP. Don't shoot the guy.
The way I see it, unless you know exactly what is going down, you aren't justified in shooting people. I'll tell you why. That last situation, where some of you, like Amused, would seemingly gladly put a bullet in guy, could go down in many different ways. The guy with the gun could be an undercover cop, a plain clothes cop, a store owner, or a fellow CCW permit holder neutralizing a guy who was commiting a violent crime. So, for those of you who opted to open fire, there are 4 outcomes where you killed a man for no reason. Hell, maybe its for an amateur video, maybe its for a TV show and you just didn't see the cameraman inside the shop. Maybe the gun is fake and its for Punked. There are too many variables for you to just shoot to kill someone.
So, in conclusion, if you have explicit first hand knowledge of what is going on, by all means excercise your right to shoot he bastard. Otherwise, in my mind you are not jusitifed because you are abandoning any rational judgement and just shooting. Cops rarely ever shoot to kill unless they have to, unlike some of you nuts who would shoot a bank robber as he walks down the bank steps, or put a bullet in a guy during a hostage situation. There is a reason that cops don't go in guns blazing unless they know exactly what is going on, and after they have given non-violent means a chance.