So, very few things are less interesting to me than determining when a Law of Magic is "officially" broken. Your Story, page 284, end of commentary.
What is interesting to me is the stuff the game will do when it comes up in play. So let's talk practical and leave the fictional lawyering for others.
A Word on Killing
Consider that the game rules do not, in fact, allow for an accidental or emergent result that kills someone.
The only way you can kill someone is to take them out in a conflict and then declare you killed them. As the player who wins the conflict, you absolutely can always declare that when you throw magic at a mortal or off-limits being, that you put them in the hospital, knock them out long enough for you to achieve your goals, etc. If you're going to kill someone, it's a willful player choice to put that drama into the game.
(Though, theoretically, the GM could also propose it as a compel if you have "Anger Management Problems" or whatever as an aspect. Or, someone could take you out in a mental conflict and declare that you lose your shit and roast a guy. Whatever. The point is, the dice are never going to tell you that you killed someone. Someone at the table is choosing to bring it into play.)
So whenever you're talking about a First Law violation in your game, keep in mind that there's going to have to be, by default, some sort of consensus about crossing that line.
To Power or Not to Power
At the end of the day, a Lawbreaker power is the same as the rest in one key area - all that matters is that you can somehow justify bringing it into play.
When you're figuring out how to make that decision, I'd like to suggest that the least important question you should be asking is whether it's a fact or not that the law was broken. Instead, you should ask yourself if the impact on that character makes things more interesting or less interesting, and how.
Consider the case of Joe Wizard, who blasts a guy in a fight that's about to shoot his friend and takes a compel to say he killed that guy. He takes the stunt and shifts an aspect to "I Do What Is Necessary", which becomes the justification for more killing.
Late in this storyline, some scenarios down the road, we find out the dude he axed was actually a White Court vampire flunkie working for a more powerful boss.
Does that make the process of corruption suddenly invalid? Do we rewind everything?
Hell no! The Dark Powers are always willing to help. If I've convinced myself I have a conduit to killing magic, why would they argue with me? Magic is belief. The line between "the universe is corrupting my soul" and "I believe my soul is corrupt" is super, super thin.
So regardless of the initiating circumstances, there's something to be said for taking on the power anyway - it provides a tangible benefit to using that kind of magic again, which puts a new set of choices in front of the player every time he takes his stuff out for a spin. It puts a temptation out there for a free benefit if you're just willing to declare your magic inherently, unremittingly lethal.
Keep in mind, too, that there's nothing technically stopping you from spending one of your actual milestones on a Lawbreaker power, if you later decide that an "edge case" incident counts as a Lawbreaking as far as your character is concerned.
Aspects Work By Themselves Too
Understandably, though, this isn't always the effect you're going for. Sometimes, maybe you just want to give Joe Wizard something to think about or reflect on, an attitude adjustment, personal reflection, etc. Maybe the point is that he *almost* went over and is pushing at the boundaries of Lawbreaking with an edge case.
Every session gives you the opportunity to rename an aspect. If you want this in your game, all you have to do is change up an aspect to reflect it. Anything that happens in the story of the game can be given tangible weight by reflecting how it affects the character through renaming aspects.
This is a good option to take if, for some reason, there is serious disagreement about whether the violation is "real", but you want it to have an in-game effect anyway. So if you can't reach the consensus that says, "this is a Lawbreaking", your character can still go through "Loss of Faith in My Magic" or "Overcautious About Battle Magic" or whatever you want.
Of course, I think you get the best mileage with the Lawbreaker power *and* a changed aspect, but that's just me.
Corruption Isn't Always About Evil
We have a tendency to look at the Laws as things that turn ordinary, nice wizards into MFing Kemmler. So, it's understandable that some players are going to have an issue with the idea of being a Lawbreaker, because they don't really want their character to be an Evil Jackass.
But all we really know, as a baseline, is that breaking the Laws fundamentally changes you somehow. There's a lot of room to decide how you're going to express that change. That's why you don't have to, if you don't want to, worry about intent too much - good intentions can cause corruption just as much as bad ones.
Let's look at another Joe Wizard. This is a young dude, just getting started, who fries a mugger in self-defense because he's afraid. First Law violation, period dot.
But what if we decide the aspect is "Crippling, Massive Guilt"? 'Cause clearly, Joe's not a bad guy, right? No one expects he's going to go from magical self-defense to setting kittens on fire just to listen to them shriek.
However, what could happen is that his guilt keeps him from using his magic, even when its arguably necessary. Even when it could help people and prevent harm. Even when an innocent is being held up by the throat by a loup-garou, and he could save that person, but God, what if something goes wrong? What if he misses? What if he kills another innocent? Better that they die by the loup-garou's hand than his, right? Better he doesn't have it on his conscience, right?
And soon, this Joe Wizard finds himself utterly incapable of risk and sacrifice. His decisions become inherently selfish, all centered around keeping him, at all costs, from having to deal with that guilt again.
How is that not a kind of corruption?
So, keep in mind that you don't have to characterize this process as a descent into blistering, making-soup-with-babies sadism. Anything that people can feel can be taken too far and become destructive.
The World Doesn't Know the Difference
In terms of what happens outside the character, regardless of whether you express the incident with a Lawbreaker power, an aspect, or both, keep in mind that people in the setting are going to draw their own conclusions.
I mean, Morgan came after Harry in Storm Front for a crime he didn't even commit, and then came after him again for investigating it because of how much of a hard case Morgan is about the Laws.
The White Council doesn't have some mystical way of knowing with 100% certainty who broke the Laws and who didn't. And like any other organization, they turn a blind eye toward Lawbreaking they deem necessary to achieve their goals (see the Blackstaff).
So, whether you choose to take the power, change an aspect, both, or neither, it doesn't say anything about what's going to happen in the story relative to the incident in question. You could all agree that no Lawbreaking took place, make no changes to the character, and the White Council might still come down on you because some Warden heard you broke the Law and gets zealous.
You could take the power and an "All Kittens Must Die" aspect, and have everyone else in the setting think you were totally justified and totally off the hook. And any combination of things between these extremes. Maybe that's what's actually the interesting part about the situation - not what happens to the character, but in the world around the character.
In Sum
Whether or not the Law was broken is the most boring thing to dwell on - either you reach consensus quickly or you don't. From there, move on to what's interesting. Where the drama lies in the situation, whether for your character or the setting or both, should be your guide to playing with the Laws using any or all of the options above. Have a guy take a Lawbreaker power when everyone else thinks it was cool. Have the world tell a character he's evil and broke a Law when his sheet doesn't change. Play with the middle ground and all of its awesome potential.
And don't worry about the so-called facts.