In response to Sky Masterson:
Sure, if some thug attacks you with his fists and you knife him, you're going to have legal troubles barring some very unusual extenuating circumstances.
If you carry a lethal weapon, you'd damned well better understand use-of-force laws specific for your state. And you'd better be prepared for any sub-lethal problem that comes along, either via running, unarmed combat skill, above-average strength or pepper spray.
All of that goes without saying, it's part of the legality and philosophy of armed self defense. The issues are identical whether we're talking knife or gun.
I know of two basic categories of circumstances where you *might* get away with knifing/shooting an unarmed assailant:
1) You *know* for a fact that the aggressive individual is physically and mentally capable of doing serious bodily injury to you. He's either done it to you before (often the case with battered women, if the SOB put her in the hospital the last time and now he's out of jail and pissed, well, there ain't much guesswork involved...) or you've seen him lash out in such extreme fashion against others.
2) The guy attacks you and actually delivers extreme damage and continues his attack...plausibly, you can say "enough". The classic example was the Hale shooting in TX, 1996. 50ish year old Gordon Hale was attacked while seated in his truck by a much younger, stronger "road rager" who'd hit Hale's vehicle wand was objecting to Hale's writing down the rager's license plate.
Hale suffered a broken cheekbone and detached retina. The rager then tried to drag Hale out the open window of his vehicle. Hale grabbed a legally-carried Glock and put one round through his assailant's chest, killing him. I seem to recall this was the first shooting under TX's new CCW laws effective 1/1/96 and as such, it was widely scrutinized.
Hale was no-billed by a TX grand jury, who found he had fired to prevent his "death or great bodily injury" in accord with TX law. California uses that same "death or great bodily injury" language to describe justification for lethal force.
IMPORTANT: THESE CIRCUMSTANCES ARE RELATIVELY RARE, AND ALWAYS CONTROVERSIAL. In the Hale case, the DA tried desperately for an indictment.
You must have a non-lethal alternative of SOME type available.
Jim