To the best of my knowledge, in most US jurisdictions, continuing to apply deadly force after the threat has been neutralized (by any reasonable assessment) is illegal...and that's as it should be. Deadly force is ethically justified in self-defense, but that justification evaporates when the threat no longer applies.

I've been trained to shoot until the attacker drops. That is, is no longer standing. I'm small (and an attacker is statistically likely to be a larger, much stronger male). Them dropping any weapon they're carrying doesn't negate the threat: they could kill or severely injure me with their bare hands. I'm shooting until they drop.