You are speaking of the legal theory called "Disparity of Force."
In order to be justified to use lethal force to defend against an attack, you must be in immediate and otherwise unavoidable fear of death or crippling injury.
The three components which comprise a reasonable person to believe they are in immediate and otherwise unavoidable fear of death or crippling injury is if your attacker has the ability to kill or cripple you. The attacker must have the opportunity to kill or cripple you. And finally, the attacker must have placed you in jeopardy of being killed or crippled by using threats and or deeds which makes one believe they are serious about harming or killing you.
So, all things being essentially equal. You are of equal size, build, strength as your attacker... You have to be threatened by someone who can not only kill you, but also has the opportunity to kill you and intend to kill or cripple you. So usually you can not employ deadly force unless deadly force is being used against you. That is why you are usually in deep doo doo for shooting an unarmed attacker.
However, there are mitigating factors or circumstances which would allow the defender to use lethal force to defend themself where ordinarily they would not be. One of those mitigating factors is where there is a "disparity of force."
Which loosely means, the force was unequal, so to speak. A disparity, if you will.
Some examples of disparity of force issues would be "Force of numbers." In that case, while your attackers may be "unarmed," there is a mob of bad guys who are threatening you using the force of the entire group. Five against one otherwise healthy individual would stomp that person into the ground and easily kill or cripple that person, therefore, it can be excused if the lone defender employs lethal force to defend himself against a "group" of unarmed individuals.
It is important to know and understand that the three components of them having ability, opportunity and placing you in jeopardy by their deeds and actions still has to exist.
Another example would be getting into a fight with an unarmed attacker because you bumped into him causing him to spill his beer and he hits you. You are both pretty much of equal size, stature, weight and physical capability and under normal circumstances you would not be on solid ground to use lethal force in defending yourself. Now say the guy knocks you down on the ground and once down on the ground, instead of walking away he starts to kick you and stomp on you with his boots. He's kicking you in the head and stomping on your body causing internal injuries. Here is a situation where while it started out a situation where lethal force is not justified, it has now moved into a situation where there clearly is a disparity of force. You are now down on the ground, unable to escape and not being on your feet and able to move, you can no long absorb the blows coming to your head and body. You may now be justified to use lethal force to end the fight, where before, you may have been guilty of manslaughter.
That example is a very complex one and a lot of people are going to disagree. A lot of people are going to say, I could be killed by a single punch to the face therefore, I'm going to shoot anyone who tries to punch me in the face, whether they have a weapon or not. Fine, it's your life, go ahead and blast them. I'm not going to take any bets one way or another whether that person doesn't go to prison or not.
It is generally accepted in society that women are the weaker sex of the two sexes and a lone woman against a single unarmed man can be considered a disparity of force and can justifiably use lethal force against a lone unarmed male opponent. However, there have been cases where the woman has been charged.
Another case where there is a disparity of force can come into play is if the person being attacked has a physical disability. For example, say you are an otherwise normal healthy male on par with your opponent, however you have just had knee surgery and are using crutches, a cane or wheel chair for mobility. You can not be expected to perform the same way to defend yourself against an attack as you would be were you before the injury.
The good thing is that your physical disability does not need to be visually obvious to the bad guy. However, your physical disability had better be well documented by your physician, surgeon, or whoever which can attest that you were in such a frail condition that any slight blow would permanently cripple you.
I hope this helps explain some things without confusing you further.
Look into "disparity of force" cases and defense and speak with your attorney if you have a physical disability where you think you may have to employ lethal force against unarmed attackers.
Once again, you have to clearly show that the average person with the same disability as you, would reasonably be in fear of your life or being crippled in the same situation. "The reasonable man theory." And again, you have to show that not only did the attacker have means (ability) to kill you, he had to have the opportunity and performing threats or actions to have placed you in jeopardy as well.