Glenn Dee said:
I think everyone would agree that in order to prove a self defense, saying "I was in fear for my life" is NOT going to cut it. And you may be looked at as the guy who brought a gun to a fist fight.
I don't think there is any way -- ever -- to "prove" self defense.
As you have stated, the basis of most (if not all) states' self defense laws is that the victim (meaning the person who defended himself) must "reasonably" have feared either death or serious bodily injury.
But the devil is in the details. As Frank Ettin has posted more than twice previously, the beginning of a self defense claim is an admission that you killed the assailant. In other words, you are charged with a homicide (murder, manslaughter, negligent homicide, whatever). You do NOT get to sit in court and play the "Prove I did it" game. In order to assert a claim of justified homicide for reasons of self defense, you must begin by admitting that you killed someone.
As Frank has also written, this essentially reverses the roles in court. The state no longer has to prove beyond a reasonable doubt that you killed someone; you already admitted to that. Now
YOU have to demonstrate two things: (a) that you were in fear for your life (or in fear of incurring serious bodily injury), AND (b) that your belief was reasonable.
The fact is that, in a self defense case, you do exactly what you said above: You go into court and say, "I was in fear for my life." However, as the idiot retired firefighter in Texas (?) found out recently, those words are not a magic spell, and they do not automatically result in your getting to go home. There is probably no way you can "prove" something as ephemeral as a state of mind, but you CAN do your best to convince a jury that, under the same circumstances, any reasonable person probably would have been in fear for their life, and therefore the jury should accept that it's reasonable for you to have been in fear for your life.