Spexxvet, enough is enough when the bad guy is lying there with a third eye and no brain wave. If you can arrange that. You can't always, but when you can, that chap's troublemaking years have come to a sudden end, and that's what's wanted.
At least by non-hoplophobes -- who seem to be the people with sense on their side in such matters.
Raging against self defense never works.
Particularly on those who prefer self-defense to death.