The COURTS decide, obviously. I'm sure US judges have examined this question quite thoroughly, and I expect they have reached conclusions similar to British judges. I believe it involves deciding whether an ordinary person of normal fortitude would aprehend death in similar circumstances. Of course, we rely upon the sitting judge to make that decision. Fortunatley, they are trained to do so.

I agree your knife man is more threatening than my gun carrier, who would probably not make me fear for my life. But there is a line to be drawn, and I believe it must be drawn earlier than you appear to.


I would be quite content to go through life without having to deal with such a situation.
How many Americans do go through life that way? I know that the vast majority of Brits do, in an apparrently much more violent country.

But just because an attacker isn't armed doesn't mean he can't, or won't, kill you. Self defense means protecting yourself, as much as is necessary. Naturally, if someone attacks you and you shoot him in the arm and he then runs away, continuing to fire on him until he's dead would be criminal. But shooting him until he stops attacking is justified.
That is why the law follows the doctrine of proportionate force. And it is why I deplore and despise the American authorites' encouragement (for that's what it is) to use lethal force as a first resort and to protect property (see den's post above).

Carrying a weapon if you know the chances of being attacked are high (greater than even, I suggest) could be argued for - but so could avoiding the situation completely. But carrying a lethal weapon against the remote chance (less than even) is much harder to justify, especially when avoiding the situation is still an option. That's not truly self-defence, it's suppressed agression. It's saying, "If you do anything to upset me, I'm going to kill you"

(If the weapon is concealed, the carrier's attitude is the same, but no warning is given.)