Terry Pratchett said this quote

“ There was a wicked ole witch once called Black Aliss. She was an unholy terror. There's never been one worse or more powerful. Until now. Because I could spit in her eye and steal her teeth, see. Because she didn't know Right from Wrong, so she got all twisted up, and that was the end of her.The trouble is, you see, that if you do know Right from Wrong, you can't choose Wrong. You just can't do it and live. So.. if I was a bad witch I could make Mister Salzella's muscles turn against his bones and break them where he stood... if I was bad. I could do things inside his head, change the shape he thinks he is, and he'd be down on what had been his knees and begging to be turned into a frog... if I was bad. I could leave him with a mind like a scrambled egg, listening to colors and hearing smells...if I was bad. Oh yes. There was another sigh, deeper and more heartfelt.But I can't do none of that stuff. That wouldn't be Right.She gave a deprecating little chuckle. And if Nanny Ogg had been listening, she would have resolved as follows: that no maddened cackle from Black Aliss of infamous memory, no evil little giggle from some crazed Vampyre whose morals were worse than his spelling, no side-splitting guffaw from the most inventive torturer, was quite so unnerving as a happy little chuckle from a Granny Weatherwax about to do what's best. ”