Some things are too vile even for a villain to do. Some evil character crosses a line that another evil character considers to be too evil, even for them. 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;