You can't fuel your magic with negative emotions without it changing you as well.
So, the Unforgivable Curses in Harry Potter follow this kind of logic. After Harry casts a Cruciatus Curse fueled by righteous anger, it has a neglegible effect on Bellatrix Lestrange.
‘Never used an Unforgivable Curse before, have you, boy?’ she yelled. She had abandoned her baby voice now. ‘You need to mean them, Potter! You need to really want to cause pain – to enjoy it - righteous anger won’t hurt me for long – I’ll show you how it is done, shall I? I’ll give you a lesson –’
The key here is that it's not just anger that drives the curse, but the cruelty as well. That level of hatred and cruelty inevitably corrupt the person trying to harness it. If you don't truly feel it, you can't harness it, but if you truly feel it, it begins to take hold of your life until it consumes you.
This is also why the Sith form Star Wars appear to grow so powerful so quickly.
Luke: Vader... Is the dark side stronger?
Yoda: No, no, no. Quicker, easier, more seductive.
Because the use of curse magic reinforces the cruelty of the user, it becomes easier and easier to do, making the user very powerful but also sealing them off from the good things in the world.
It's pretty easy to imagine why a society would be against that. You can only handle so many dark wizards before you start taking steps to prevent them from being created.
It's the same logic our society uses with dogs. If a dog bites someone, more often than not it's euthanized, not because of the bite itself, most are superficial damage at best. It's because when the dog finds itself in a similar situation again the dog realizes that option worked before, so it's clearly on the table.