I recall Harry saying it in the books as well.
The problem is that this doesn't explain why doing other horrible things doesn't corrupt you as well. You're going to have a hard time convincing me that (for example) slowly ripping someone's eyes out to get information has less of an impact on the wizards mind then blowing their head off with a fireball.
You can also kill lots of beings without souls/Free Will and you won't get corrupted either (at least not in the black magic sense). Given how similar and friendly Little Folk are to humans, it seems strange to me that killing them doesn't warp your mind just as much as killing a human. Maybe you could make a argument for a Red Court vamp or ghoul or whatever but the Little Folk? Or an angel?
I'm not saying it's completely wrong but I think there's something else causing the corruption as well. I'm partial to the RPG theory that the first few Laws are based around Free Will and the last few are just "wrong" in the sense of "Things That Man Was Not Meant To Do".
Torture for information and other mandane ways of doing evil i.e: violating free wil carries its own taint. In PG, Murphy admits feeling tainted when she shots agent benton.
The point is, when you add magic to the equation, the taint becomes much, much worst.
It makes sense. In PG, it is stated that the reason god gave human the 3 swords is to balanced the enormous advantages the supranatural have over the vanilla human. Providing an extra penalty for wizards that violate free wil does make sense for the balance.
Wizards are humans, they have free wil. If they choose to use their free wil to kill and enslaved, it is their choice, so long as they pay the penalty.
If a predanatural creature violates free wil i.e" eating people, they cannot pay the penalty by losing their humanity/sanity. They are not human in the first place. These creatures are penaltied by different means. The little folk are weak and cannot do much harm, but they are virtually imppossible to find without magic. Lesser fei i.e: Bridge trolls, are teritorial and can easily be avoided or chase away, even by vanilla methods. If humanity choose to stay in denial and refuse to believe in the supranatural and therefore caught off guard, it is the human's own fault.
Other predators like the rampires and the blampires are the real evils. For these creatures penalty comes in the form of weakness to sunlight, weakness against faith based magic and inability to cross threshold. And if they got too active, they'll have either the KotC or some wizards hunting and killing them.
Demons/creatures from the nevernever cannot cross without being summoned. And those creatures who is powerful enough to cross without invitation i.e" Mab, have an equal power balancing them i.e: Titania.
All in all, Butcher have created a quite balanced and realistic supranatural world.