There is not 100% certainty in a regular self defense situation.
Bad guy is attempting to kill your family.
You decide to stop him.
Your family is dead.
You attack.
Bad guy is dead.
You go to jail for murder, because you attacked too late (not self-defense anymore).

Your waiting for a 100% certainty (because clearly there were more unseen consequences) would end with a worse situation. The mother fucker got away.

(I didn't know there were so many people against the death penalty here: "Nobody's really bad!": poor mislead souls.)