I also believe this is why you go mad/have to be mad to mess with time, kill with magic etc. You have to believe in complete violations of the Laws of Reality.
How is killing someone with magic violating the Laws of Reality more than say, blowing up the building? To me, both seem equally "impossible", if you want to apply, let's call it "atheist" way of thinking (as in, "Magic does not exist!") and equally possible otherwise.
I think I've read somewhere, that it's the "slippery slope", that's a problem, evil (be it killing, or manipulating minds, or other stuff) gets easier, and eventually you find yourself a warlock (like with the guy who was executed early on in "Proven Guilty", or like how Harry explains why messing with people's minds using magic is said slippery slope, also in "Proven Guilty").
Plus (I think) it only seems to apply to humans for some reason - in "Changes", Harry flattened probably at least a few dozen of Blood Court vampires (by the end, when he did the gravity trick), without, as far as I can tell, any mental consequences.
Not to mention destroying the whole Red Court later on...
(well, unless there are consequences later on, and I just didn't get there yet)
Though I now wonder, if Winter Knight's Mantle had anything to do with it. Still in "Changes", I recall it showing Harry a mind-image of (I think) Rudolph frozen to the wall, or maybe impaled on some massive icicles. It didn't seem to have any actual control over him (so far?), but maybe it does some subtle influence, kind of like Lasciel's shadow did? (but don't tell me, I want to read about it for myself)