The legal test is "would a reasonable person, in the same situation, feel the same as the defendant claims, at the time of the incident"
You are not allowed to assault me. Just because you do not have a weapon does not mean you can punch me and break my jaw.
Another legal standard. "If I am in a place where I am legally allowed to be, I am allowed to respond with deadly force against any threat which endangers myself, a loved one, or any member of the public. Be it harm or death, from occurring."
There is an old saying, 2 actually, that apply here.
I am too old to catch an ass kicking, and too young to die.
and second.
I would rather be judged by 12 than carried by 6.
Fear, genuine, real fear, is the cornerstone of a legal shooting. You are NOT allowed to harm me, my family, or anyone around me. I am in a place I am legally allowed to be, engaged in legal activities. You have just threatened me with "death or great bodily harm" (those are legal definitions, look them up if you like) I can respond with deadly force to stop the situation before it escalates further.
In fact, up until the early 1960's police and US Federal Agents were allowed to shoot someone for simply threatening them.
"fuck you cop, Ill kill your ass if you come over here"
Bang
"I was threatened"
To this day, simply being in a place you shouldn't be, IE, illegally, is enough to justify deadly force. Be it a military base, or someones home. No threat is necessary, just the trespass.
I could go on for days. If you are genuinely interested, I will keep posting.