If you reasonably believe it is necessary to do so to prevent death or great bodily harm to yourself or another.

Killing some random grandma for showing you middle finger does not really qualify.

However killing threatening idiot armed with something and facing you is totally different. To note - if idiot hits you and then runs away - sending bullet to catch him is not a good idea and not going to work (unfortunately).

On a final note - in a state with good CCW laws people typically are more polite and well behaving. Flipping a bird to random asshole might backfire and it won't matter to you whether he knows the laws or not.

Laws vary between areas but the general approach is that you can only respond to force with equal force. It's obviously more complex than that but that's what most laws are based on.