First of all, not all of the Laws are about evil; the last three (no necromancy, no time travel, no summoning Cthulhu) don't make you evil, they just tend to lead to Bad Things happening as a side effect.
For the first four Laws, it's all about intentions. Dresden Files magic relies on belief and state of mind, and mind-control magic is evil because (A) to perform it you have to completely
believe that your own will is more important than your victims' and (B) using it leads to using more of it, which causes a vicious cycle of evil. As Dresden explains in Proven Guilty:
Quote:
“It…” I frowned. “Getting into someone’s mind like that is difficult and dangerous. And sooner or later, while you’re changing them, you start changing yourself, too. You remember Micky Malone?”
Murphy didn’t exactly shudder, but her hands stopped moving for a minute. Micky Malone was a retired police officer. A few months after he’d gotten out of the game, an angry and vicious spiritual entity had unleashed a psychic assault on him, and bound him in spells of torment to boot. The attack had transformed a grandfatherly old retired cop into a screaming maniac, totally out of control. I’d done what I could for the poor guy, but it had been really bad.
“I remember,” Murphy said quietly.
“When a person gets into someone’s head, it inflicts all kinds of damage—sort of like what happened to Micky Malone. But it damages the one doing it, too. It gets easier to bend others as you get more bent. Vicious cycle. And it’s dangerous for the victim. Not just because of what might happen as a direct result of suddenly being forced to believe that the warlock is the god-king of the universe. It strains their psyche, and the more uncharacteristically they’re made to feel and act, the more it hurts them. Most of the time, it devolves into a total breakdown.”
Same for killing, shapechanging others, and mind-reading. Giving a violent/unstable/inexperienced/etc. person the freedom to spam
dominate person,
detect thoughts,
finger of death, and
baleful polymorph is a Bad Idea by itself, and the fact that casting them makes you want to cast them more just makes things worse.