For me, storytelling is all about character development. Character is the essential, crucial element that captures and holds my interest in whatever I am reading. In many story lines, the villain is an irreplaceable, integral component. The desire for power will always be the controlling factor in the existence of any society: Human, animal, or supernatural, there is always the quest for dominance.
Evil is insidious, but it is never simple, or just black and white. The more layers and shades of gray it obtains, the more horrific and invasive the evil becomes. Most humans have a touch of evil. It may be just a flicker, but it’s there.
True evil seeks out that tendency for evil in others and uses it for its own dark purposes. In fiction, good fiction, at least, even the supporting players, the not- so-nice people, and especially the villains should be well-drawn and multi-layered. Minor characters, sharply etched in a few well-chosen words, add such rich flavor to a story line. I like nice characters who have an unexpected naughty streak. I love naughty characters who are nice when they least expect it themselves! I like characters who are basically defined around the edges, but still flexible enough to be surprised and revised. A “complete change of character” is not really believable, and it's also not very interesting. It's the little flaws, “uh-ohs,” and “ahs” that make for a readable character.