Fan Casting Even Evil Has Standards in Sort by Tropes

One of the easiest ways to highlight just how bad something or someone evil is: have an otherwise-remorseless villain reject it. It's often to show that a new villain is really bad if even Doctor Annihilation is appalled by them. Another way that it's used is to keep a villain safely on the "still sympathetic" side of the Moral Event Horizon; give him something that he simply will not do. It may be specifically invoked to prove that it's OK for our hero to work with villains who have standards when the need is great enough.