When performing a killing blow, you deal damage equal to your full dice pool plus your weapon modifier.
You’ve time enough to line up your attack so it avoids your victim’s armour.
While people who kill in combat can justify their actions based on the heat of the moment, performing a killing blow is a premeditated attempt to end a sentient life without the target having a chance to do anything about it.
Going through with a killing blow is an Act of Hubris for mages with Enlightened and Understanding Wisdom, whether the victim survives or not.