The moral of much series fiction is that the villains are irredeemable - because they have to return as villains in later episodes. I am interested in the processes by which people, including you and me, not just fictional characters, can or might change for the better:
Fr Brown converts Flambeau;
in Smallville, not only does the friendly Lex Luthor become ruthless but also his amoral and manipulative father, Lionel, becomes concerned and self-sacrificing;
Poul Anderson's Aycharaych loses the motivation that had made him an artist of deception and manipulation;
SM Stirling's William Walker does not see other people as real but suppose something did happen to make him "realize" one of his victims and start to care about what he was doing?