I' with Buzzard on this one. If a player wants to play a lawbreaker, he should take the appropriate stunt. I mean... how would you kill a little bit?
Also, remember the lawbreaker stunt has nothing to do with the White Council. They will kill you if they think you have it, so you can have a stunt and be fine or don't have it and lose your head.
And lethal force doesn't have to be lethal at all. The important thing is the taken out result. I would never have someone take a lawbreaker, because he shot a fireball at someone and took him out, but if he then did something to ensure the death after that, that would come with a lawbreaker stunt. But again, the player actively decides to kill someone, it isn't as though he rolled extremely high damage or something, he just WANTED someone to die by his own hands, and that's what the lawbreaker means and should mean. Otherwise it would be pointless.
However, there can be occasions, where there is no need to take a lawbreaker at all. If for example everyone in the group is a lawbreaker, I would simply have them one lawbreaker for free. There is nothing special about it in a group like that, so I don't see a reason to charge everyone for it.
If your group has a problem with the lawbreaker idea, you could simply tweak your dresdenverse to make magic not corrupt the wielder. That way, you can play how you like it without having to worry about the things you don't.