The First Law of Magic in the Dresdenverse states that you cannot kill mortals with magic. The White Council gets pretty uptight about this, so it is a Law worth following.
In the Dresden RPG, generally if you take out an enemy in combat, you get to choose how they are taken out - i.e. dead, unconscious, captured, whatever.
So does this mean that, if I'm a wizard PC, I don't have to worry about violating the First Law, because if I happen to take out a mortal, I can just decide that I took him out in a non-permanent fashion?