Not because of the insult, but because they were behaving in a manner that indicated extreme irrationality. Unfortunately, to wait until I'm actually attacked would essentially be to bet my life on the other person's sanity--which, as I've pointed out, is demonstrably gone.
"He who shoots first writes up the incident report. He who shoots second is likely to never get a chance to shoot at all."
There are certain things that I don't do because they are likely to convince people that I am out to kill them. Screaming obscenities in their face is one of those things.
In most states I am aware of, the threshold of lethal force is imminent threat to life or limb. "Behaving in a manner that indicated extreme irrationality" won't cut it, unless of course you're not one of the 'little people'. Then its just a game of lords and peasants, which is what we have in this article.