The use of deadly force

To the unprepared, the shock of sudden and unprovoked violence has a tendency to paralyze the victim with fear or cause doubt in their mind that violence is being committed against them. They are often unable to react or wish it was not happening. Understand that you fight to stop, not to kill. However, you may end up killing or maiming. If you have not made peace, in advance, with that decision, you may not be able to make that decision when you need to. Should you ever find yourself in a lethal confrontation; the decision to use deadly force is going to be yours and yours alone.
This why you must decide ahead of time how much force you are willing to use to defend yourself, your loved ones, or another innocent.
You are universally justified in the use of deadly force when there is a reasonable fear of immediate or otherwise unavoidable danger of death or serious bodily injury to the innocent. All of these factors must be present to justify the use of deadly force. If one of them is missing, you will most certainly face civil and maybe even criminal charges.
You or the person you are defending must be free from fault. In other words, you can not start a fight then claim self defense.
You know when you are in immediate and unavoidable danger when your assailant has the ability to cause harm, has the opportunity to inflict harm, and is intent, as shown by actions or words, on killing, crippling, or permanently disfiguring you.
Lethal force is justified when there is a disparity of force such as:
A large man against a small man
Able bodied against disabled
Man against woman
Two or more on one
Someone trained in martial arts against an untrained person
When do you retreat?
In order to justify the use of deadly force, you must show that there was not an alternative. In some jurisdictions, retreat is required. If you do not retreat until you could retreat no further, then you are not justified in the use of deadly force. In the universally accepted use of deadly force, retreat is always a good idea and should be done if it is a viable alternative and does not place you or those around you in greater danger. Retreat will establish that you did everything possible to avoid the confrontation.
Always keep in mind: if it’s not worth dying for it is not worth fighting for. If you must think about fighting, you probably should not
Paraphrased from an article by Frontsite Firearms Training