Nobody is born evil (well, except maybe the Enfant Terrible). Something usually happened to push a villain down the path to villainy. And hey, mightn't that make an interesting story? Thus, we have the Start of Darkness, a Prequel Dark and Troubled Past where we find out how the main antagonist from the original story got to that point. This, naturally, is especially common with Fallen Heroes, who usually get a Downer Ending where they lose faith in themselves and/or humanity. This will be especially poignant if they Used to Be a Sweet Kid (see also: Freudian Excuse).