The law varies from state to state, but where I'm from, before you shoot someone, three things have to be present at the moment you pull the trigger. If any one of them is absent from the situation, you've just gone from self defense, to murder. Those three things:
1.) Ability: The person has to have the ability to cause you harm. A person may want to kill you, but if they don't have the skill or physical ability, you can't shoot them. You're expected to use less lethal means to deal with them. Escape is preferred over taking of a life.
2.) Opportunity: A person may pose a threat to you, but if they have a knife and they're so far away that they can't stab you with it, you can't shoot them. If you're riding by in a car and someone is chasing you with a baseball bat or something, they may have the intent or the physical ability, but they won't have an opportunity. If you can avoid the bodily, you are expected to do so.
3.) Jeopardy: Just because I have a hand gun or because I'm 6'2 tall and I have the ability to beat the life out of you with my bare hands, that doesn't mean you can just shoot me. Yes, I have the ability to cause you harm, and I have the opportunity, but I don't have the intention of causing any physical, life threatening, harm. This is the hardest one to prove and the reason why, even law enforcement officers that shoot someone have to go to court to justify whether they did or did not use unnecessary force, If someone is fleeing, you can't shoot them--even if they're a murderer. The only exception is if you're trying to protect someone from a "Forcible Felony"--being raped, being robbed, or any other circumstance that will cause them serious physical harm.
So, even though a lot of people have weapons in the U.S.A., it's not the wild west. Even pointing a gun at someone is illegal without a legitimate reason. Just having a gun during the commission of a crime is a felony. Youcould be selling drugs or breaking into a house. You don't even have to use the weapon to be charged with possession of it. Before you un-holster a weapon, you have to think about whether or not YOU want to spend a lot of time in Federal prison!
The thing is, if you shoot someone and kill them, you're still alive to tell the circumstances of what happened. A person can be in the right when they kill someone but they can't prove that the person who was shot had ALL THREE factors (Ability, Opportunity, and Jeopardy). and they can go to prison. That same person can shoot a guy coming at them with a beer bottle. If they can prove to a jury that the person was physically imposing enough to kill them with a beer bottle, had the opportunity to do bodily harm, and they were in reasonable fear of their lives, they can be found innocent. The saying goes: "I'd rather be judged by 12 (aka the jury) than carried by six (aka funeral pall bearers)."
