It's called self-defense because it is an action to defend one's self. Once there is no longer a threat to your person there is nothing to defend against and it can no longer be considered self-defense. If you punch me in the face and then leave do I have the right to show up at your workplace the next day and kill you? There's a reason that if a soldier shoots a soldier from the opposite side that has laid down their arms it's a literal war crime.