This is bullshit.
Anyone watched the movie "Wolf Creek" recently?
The two girls would have escaped death if they whacked the villain
while he was injured and unconscious on the ground. But no, they
let him live and eventually he hunted the two girls down and
slaughtered them like animals (actually worse than animals).
I am not saying that we should hit back at our attackers with the
intention to kill. But, when you are being attacked and you gain
some kind of advantage for a second, it is perfectly logical that
you will try to make use of that advantage to further weaken your
attacker SO THAT he is not able to recover immediately and ATTACK
YOU AGAIN.
Self-defense doesn't mean just being passive and try to block every
blow from the attacker (no human in this world can block every blow
from anybody). You will get killed eventually if you let your
attacker repeatedly assault you. Self-defense is trying to take
away your attacker's ability to continue assaulting you, and
sometimes the best way to ensure that is if the attacker is injured
and stopped.