Because you have your character heroically fighting against the crazy guy with the ax (or something like that) and then the GM says "Ok, now you're an evil murderer and you have to change your personality to reflect that", when you (and your character) are absolutely certain that you aren't in the wrong. Or even the GM actually taking your character away from you for trying to be a hero.
Hmm... maybe I could make this work by imagining that the Lawbreaker bonus in this specific case reflects less homicidal mania and more the sort of psychic conditioning that soldiers get to train them to fight and kill when necessary.
First, no, you're grossly exaggerating the effect and the immediacy. It's not an instant change, any more than smoking one cigarette instantly turns you into a chain smoker. There's a reason the Lawbreaker power has it so it changes the more you break that law--one kill with magic isn't going to turn you into Bloodbath McEvilwizardington. But one kill with magic will make you somewhat more likely to kill with magic again, and it's a cumulative effect.
Secondly, you seem to be operating from the impression that a character
has to kill (or even, oddly, that they have to kill to be a hero?), when that's pretty much never the case. Outside of a compel, it's always the player's choice how a Taken Out result goes.
And if you do kill once with magic, it doesn't ever mean "Ok, now you're an evil murderer and you have to change your personality to reflect that," it means, "Okay, now you're the sort of person who believes they can kill with magic, change one aspect to reflect that."
Here's the key words: "when you (and your character) are absolutely certain that you aren't in the wrong" is
exactly why there's the laws of magic. Some of the worst atrocities in history were caused by people who were "absolutely certain" that they're not wrong. It's exactly that mindset--"I'm completely right for killing this person," via lawbreaker, becomes, "I'm right for killing," and eventually works itself up to, "I'm always right for killing, and you deserve to die because I said so."