I don't know. Personally, if I were running the game as GM, I would make it so that Lawbreaker stunts are only taken in cases in which a character intentionally breaks a law of magic, as opposed to unintentionally: eg if you were throwing a lightning bolt and it accidentally struck a civilian you had no way of knowing was there, that wouldn't be lawbreaker, but if you burned down a building knowing that there were civilians in the building and some of them died, that would. In the same vein, if I didn't know that a player remembered that breaking a law of magic could kill their character, I would gently remind them of this fact before they acted, and if they still chose to do so, so be it.
As for whether its a 'metaphysical law' of the Dresdenverse, I don't think that's exactly true. We have to keep in mind that both the books and the RPG are both written from Harry Dresden's very very biased viewpoint. He's gotten better about it throughout the series, but he still has a strong tendancy to try to paint the world in black and white. If the Laws of Magic were so clear and fundamental, it would leave questions such as:
How are some powerful wizards like the Blackstaff or the Gatekeeper able to "keep their souls from darkening" if its such a great metaphysical law?
Before Merlin created the laws of magic, did they exist/still blacken your soul?
Then there's the bit in Turncoat where Molly is tempted to look into other people's minds and poke around because she had broken that law of magic.
That said, I think there needs to be a bit of leeway over the intent. When Dresden burnt down that house to kill the vampire the poisoned humans were probably already dead (and if not they were dying). If any had been alive, well, Dresden didn't mean to kill them and metaphysically he didn't break the laws of magic, but if a certain Warden had been around then I could see Dresden getting his second trial - this one ending in death.
Another thing to remember is that while the game maps well to the books, the books weren't written with the game in mind. Jim Butcher writes stories to entertain us and while he more or less keeps things at consistent he uses plot devices were needed.
Richard
In the novels, Dresden is informed by Bob that they are still mortal, just really, really messed up.
They may not have control of their faculties, but they still have a soul, which is what the 1st Law boils down to RAW.
What We Know: A Renfield is a victim of mind control magic, specifically the sort practiced by Black Court vampires. The victim is crushed into total thralldom by brute psychic force; this destroys the victim’s mind, leaving him no good for anything but gibbering violence. In many ways, Renfields aren’t people anymore.
Unlike most thralls created via mind control magic, being turned into a Renfield may be irreversible. (Even the original Merlin was unable to undo the damage.) Renfields get more and more deranged and violent over the course of a year or two, and eventually self-destruct.
Powers: Fanatic strength and pain ignorance. These are normal folks, built along the lines of rough thralls (see page 82). Add Inhuman Strength and Inhuman Recovery, and you’ve got the package.
They are just enhanced humans that are "mindless".
So I tracked down Strange Brew, a collection with a Dresden story in it. It might not show how Wardens act, but it shows how they are believed to act.
"That’s the upside of working with the gray cloaks now, Burt," I said. "I don’t need proof. I just need an excuse."
Seeing that, and Mac's "Was it you?" question in Proven Guilty, says a lot about how much evidence the Wardens seem to need.
Richrd
Actually in the first book or two he does mention that you can see (or feel) the taint on someone when they have broken a law. This is one of the few things that has not been consistent about in the books or black council, sorcerers, etc.. would be obvious once you get close enough to them.
Though really even if you are SUPER EVIL INCARNATE that doesn't mean that you have to break any laws. In fact the extremely dangerous ones (i.e. the sane ones) probably avoid breaking the laws as much as possible. Cowl would probably be a good example.
Another thing after seeing the effect that the blackstaff has wouldn't be surprised if we find out that it is actually alive and is actually a part of the "original owner".
If the Laws of Magic were just things that people had written then you wouldn't be able to See lawbreakers. I could be wrong, but if I recall correctly when Dresden and others viewed Molly in Proven Guilty they could see that she had broken the law. They needed to Soul Gaze her to know how bad she had broken it, but they could look and See that she had tampered with others.
Also Molly was twisted to the point where in Turn Coat it was second nature to her to try to mettle with an unconscious wizard - after years of apprenticeship and knowing that if she was caught she and Dresden would both die she did it anyway. That's an important point - Dresden had drilled and drilled and drilled "Do that and we both die" into her and she still tried to do it because of that lawbreaker taint.
Richard
She did it twice - twice in total, Years later, after training and daily reinforcement not to do it (or we both die) she does it again. How is that changing over time? If anything she should have changed into someone who would never used magic that way.
Every place in the books when they talk about the Laws of Magic they refer to the damage done to the person breaking the law. In the gaming books they do the same. Even in OW under thralls there's a note that says: "If you change others you change yourself".
Now if a group wants to change things then that's fine, but the baseline given in the books and the game is clear - breaking the laws of magic twist the law breaker. They leave a mark on the law breaker. Break the law once and you're always fighting temptation to break it again (even Dresden has to deal with that over killing his first teacher). Break the law enough and you become a monster who must abuse your power.
Richard
Lots of grey area involved in this discussion, especially with respect to the first law. I think they way I would adjudicate this is to look at motivations to clear up the grey areas. I think that in most cases a use of magic that leads to a mortal death falls into one of several cases:
1) Magic was used to directly end a life.
2) Magic was used in the expectation or hope that a life would be ended.
3) Magic was used recklessly without regard to the possibility that life might be endangered.
4) Magic was used as responsibly as possible, but something unpredictable within reason occured.
Generally, the first three would be considered lawbreaking, and the fourth would not, in my opinion. So teleporting someone over a volcano or into a deadly part of the Nevernever would be #2. Blowing up a building without even considering that someone might be there would be #3. Tossing a fireball at a vampire just as someone teleported directly into your line of fire would be #4. Wardens are certainly risking #2 through their work, which makes me think that perhaps their swords offer some measure of blackstaff-like protection. Perhaps instead of the blanket immunity it offers protection only for warden-sword executions in defense of the laws of magic.