Deadly force may be used only when there is an immediate and unavoidable danger of death or great/grave bodily harm to an innocent person, where no other option exists other than the use of deadly force.
There are shades of gray on everything and there are three important distinctions.
The harm of death or serious injury is immediate, unavoidable, and dangerous. The situation is happening now; you cannot walk, run, drive away, or take cover behind some type of barrier that would keep you safe from bullets, fists, sticks, or knives; and that the encounter is something a "reasonable person" (a term used frequently in court to justify legal behavior) would find dangerous. Here, we define a "reasonable person" as any sane, adult member of the community, not a cop, special warfare operator, lawyer, psychologist, social worker, defensive tactics expert, firearms training expert, or a high-ranked martial artist. A "reasonable person" is you and me and our neighbors and fellow citizens.
To draw your weapon and fire, you must be in fear, and it's happening now and not last week or next week. You must feel (and be able to explain later) how you thought you could lose your life or be seriously/gravely injured (the kind you may not recover from, or die later because of, or the results of which would impact your life, drain your blood, leave you with a permanent limp, or give you a scar caused by a bullet, sharp item, or heavy object). With those standards surrounding the situation, you can fire to protect yourself or someone who may be a spouse/partner, family member, friend, or even a stranger.
You must be an innocent party.
This means you can't do a drive-by shooting at your Neighbor the Bully as he's out front watering his lawn. You can't be participating in a crime. You can't shoot at people who are trying to steal your drug stash.
There must be no lesser force that is sufficient or available to stop the threat.
This point and the next one are interrelated. Another point that most (non-gun) people (and lawyers) don't understand is that you have the right to use a gun to stop a knife-wielding attacker. You can't use a gun to stop someone who is screaming at you, but there is no requirement to match fire with fire. You must be able to explain why you chose to use a firearm because your attacker was: bigger, stronger, in better shape, obviously on drugs, mentally ill to the point of being out of control, or would not stop chasing, approaching, grabbing, hitting you, or threatening you with an object that could kill or injure you.
You must have no reasonable means of retreat or escape.
This point is the second half of the previous one. If you can walk, ease past, dodge, run away from, drive away from, or leave via a waiting helicopter, then you can't use deadly force against the attacker. This is probably one of the toughest ones to defend. Attorneys and others believe you should have been able to levitate yourself into the air and fly away, run off at Olympic medalist sprinter speed, or snap your fingers and disappear. If you are blocked in by walls, parked or moving cars, oncoming traffic, the attacker's friends, or you're standing with your heels against a water source and you can't swim, you can fight back.
The attacker must have indicated his or her intent to cause great bodily harm or death to you or someone else. Some ways an attacker might show intent would include deliberately pointing a weapon at you or stating his or her plan to kill you.
The aggressor must have a conventional or unconventional weapon capable of inflicting great bodily harm or death. Guns and knives are not the only weapons that meet these criteria. Common objects can be used as weapons. Additionally, an attacker may be able to inflict death or serious injury using only his or her hands or feet. If the physical differences between an attacker and his or her intended victim are so great as to make it clear that the unarmed attacker could cause death or great bodily harm, the potential victim can use deadly force to stop the attack.
We shoot to stop the threat to our lives.
The subject must have a delivery system – a means of using the weapon to inflict harm. A person armed with a baseball bat, having stated his or her intent to kill you, does not meet the criteria for an imminent threat if he or she is standing 50 yards away from you or on the other side of a fence.
That's why your statements, depositions, and testimony must clearly spell out that you were afraid you or your family members were going to be seriously injured or killed by the suspect, and that you had absolutely no other choice but to defend yourself from a person who instigated the fight.
Keep your head down and keep the faith,