Yeah i'm having trouble keeping track of all this so i'll throw my own thoughts in here on the bits i am following.
1. We've seen with the warlock at the start of PG that certain types of magic do affect people, we've got a number of other such mind altering quotes as i recall too from various sources.
2. We know from a WoJ that the results matter far more than the intent.
3. We've no information on non-magic violation's, but i'm guessing they don't have the same effect.
Anyway my theory on what's happening here:
1. Magic is on some level a cosmic force, when you use it to achieve something you create a link between yourself and everything affected by what you've just done.
2. Whilst it's not clear why it's the case, affecting mortal's with magic seems to have effects that aren't present with non-mortal's. Almost certainly connected to the whole violation of free will. Presumably some kind of counterbalance to offset what's just happened. Using a cosmic force to violate free will is a sort of cosmic no-no, which seems to fit in with Uriel's actions in GS actually. Angel's, Fallen or otherwise, seem to work everything off cosmic forces, (i.e. their soulfire), and that allowed Uriel to take corrective action. In that case it was a sort of non-involuntary balancing but it could be where a cosmic force is used by a free-willed person the response is automatic. Alternatively as immortal's, Angels and the like are immune to the feedback thus meaning someone like Uriel has to step in, (and he's got less direct feedback methods he can use as a result of being sentient rather than being a form of newtons third law).
3. I think why the effect twists minds is sort of what Terry Prattchet via Vimes will often talk about, when someone kills and their sufficiently mentally unbalanced already the act of that first kill makes them feel like they have power of life and death, they feel like "a god" as Vimes put's it. So they do it again and each time they become more and more detached from reality seeing people more and more as just things to be destroyed if they get in their way. You'll hear similar things from psycholinguists about real world serial killer a lot of the time. In IRL though this only happens with unstable personalities.
I think what happens in the DF'verse with magic is that the feedback is forcibly changing how you view things. If you do something that treats a free willed being in a certain way, (intentionally or otherwise), it feedbacks to make you see free willed beings in a different way in accordance with whatever just happened.