If an attacker is down and out of the fight, that is a one shot stop. A one shot stop does not necessarily require that the attacker be terminated. We are talking about self-defense. If you are perceived to have crossed the line to start executing "judgement" upon your attacker, you may well find yourself being prosecuted. Don't put yourself in a prosecutable situation, but by all means protect yourself.
Happy to live in a "Castle Doctrine" state.