WHEN TO USE DEADLY FORCE
When there is an immediate and otherwise unavoidable threat of death or crippling injury to yourself or another innocent person!
Dissect and understand each and every word in that statement. Each word in that phrase has literal meaning to it. Learn it, know it, live it.
The two words, "otherwise unavoidable" are just as important as the rest. Live by that phrase and employ lethal force only when that situation presents itself and you'll generally be okay in all 50 states.
An "immediate and otherwise unavoidable threat of death" is generally looked at in terms of the attacker possessing the following three components: Ability/Opportunity/Jeopardy.
Ability: The attacker must possess the ability to kill or cripple you. This generally means that he has a weapon capable of inflicting lethal or crippling injury such as a gun, knife (or other edged weapon), a club of sufficient size and weight that is capable of killing or maiming.
Opportunity: The attacker must at least have the opportunity to use the weapon to kill or cripple you. For example, a gun can be used against you at considerable distance, whereas a man threatening you with a two foot piece of lead pipe, standing across the parking lot, 30 yards away does not have the opportunity to kill you with it (at least not until he closes the distance).
Jeopardy: Often the word is interchanged with intent, and means that the attacker must be actively threatening to harm you with deeds, actions, behavior or verbal threats which are believable. A man casually standing there with a holstered weapon on his person certainly has the opportunity and ability to kill or cripple anyone in the immediate area, but unless he is actually placing you in jeopardy by deeds, actions, behavior or verbal threats, you aren't warranted in responding with lethal force until said person actually places you in jeopardy by his actions and intent to hurt you.
All three of those components, (ability, opportunity and jeopardy) must be present all at the same time to be construed as placing a person in what is referred to as "Immediate and otherwise unavoidable threat of death or crippling injury". Only then would you be justified in using lethal force of your own to repel the attack.