Generally speaking, you must have a reasonable belief you are in imminent danger of death or serious bodily injury. There are several aspects to this.
Your belief must be objectively reasonable. If you are five feet nothing and a 6'6" 300# person says that he is going to kill you and initiates an attack, most people would find your belief to be reasonable. If the tables were turned, that would not likely be reasonable.
The danger must be imminent. Not five minutes from now. NOW!
The degree of injury must be anticipated as being serious. If someone says "I'm gonna kick your a$$" and moves toward you. How serious do you anticipate this a$$ kicking to be? If you are a reasonably healthy person, would an a$$ kicking kill you? Would it hospitalize you for a protracted period of time? Or, will it cause you to have a bruised hind quarter that will heal up on its own in a week or so with out any medical intervention?
If you knew you already had a broken back or had a serious heart condition that would be grossly aggravated by such an a$$ kicking, then maybe.
You may believe your actions were immediately necessary to preserve your life, but the DA and a jury may disagree. You will likely find yourself on the wrong end of a criminal trial regardless if you shoot an unarmed person.
If you are going to cary for self defense, look into a pre-paid legal service and pick the brain of the attorney that will have to defend you.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball