Being overrun by an armed mob is not a threat of death or serious injury?
It needs to be recognized that a "threat" of harm is not necessarily going to be seen as justifying deadly force -- including showing/pointing a gun in some states. In the US in general it is the "reasonable man" doctrine that governs deadly force use in self defense: If a "reasonable man" would be certain that his life or the lives of innocent others were in imminent danger of death or grave bodily harm in the same circumstance, then self-defense may be raised as a defense. When a jury gets involved, there's a whole lotta room for interpretation even in cases that might seem at first clear cut.
Especially since I got my permit and began daily carry years ago, I keep in mind every day that my #1 self-defense weapon is my brain; #2 is my feet -- get the heck outta there is always the best solution, or better yet, don't go there in the first place; the gun comes in on down the list.
If the gun comes out, it'll be the last resort.