when is it "ok"?

[TreeDoc]

As Masaad Ayoob wrote..."In the Gravest Extreme"

You're a Californian, it can make a difference State to State. It is a complicated issue but I'll give you my best uncomplicated answer that might work in general terms. You may use lethal force anytime that you feel "your life" is threatened and you have exhausted your ability to remove yourself from the situation. If a 5'6" 105 pound boney swimsuit model clad in a thong bikini comes at you with her bare hands threatening to kill you, it's not likely that lethal force would be a reasonable defense. Now that I think about it, it might be sort of fun wrastlin' with her! On the other side of the coin, if Dredrick, a 260# heavyweight boxer type has you cornered and is threatening to kill you, you think he'll make good on his offer and you're a meager 6'0, 160# computer nerd from HuntChat with no way out, you could likely drill him with a double tap and have a reasonable defense. My examples are extreme simply to point out what is or isn't a threat.

A threat by a weapon is a different story and perhaps a bit more cut and dry but that depends on many variables still. Regardless of how justified, in this day and age you will likely be tried for your "crime" so a good Criminal Defense Attorney is paramount.