When we talk about "evil," what are we really discussing? For the purpose of story villains, let's divide "evil" into three categories:
-evil, pure and simple --- conscious, pre-meditated, personified, malignant evil
-menace --- more of an inanimate evil, not conscious, involving the unknown and the primal fears that spring from it
-sophistication/corruption --- influence of the outside world
Today, I want to ruminate on the first, which is easiest to define and probably the most interesting as an author.
Let’s start with a definition. Evil can be supernatural --- the word definitely has that connotation --- but doesn’t need to be. It includes an unnatural desire --- some might say lust --- for any or all of three things: power, possessions, and control over other living things (sometimes, too, evil includes a desire to corrupt and bring others down to the same level as it has corrupted itself). Now, just about all of us want power, possessions and control to a certain degree at some time or another, but villains are really obsessed with them, usually on a massive scale. Why? That's a question people have agonized over for millennia. Trying to simultaneously rationalize and admonish, the Christian church has, through the centuries, presented us with its list of Seven Cardinal Sins (i.e. worst of the worst) as both explanation and warning: lust, gluttony, greed, sloth, wrath, envy and pride, which is usually considered the most awful. With due respect to the church, I’m not sure we have to go into all that detail, and I humbly submit that all villains, all sin, spawn as the result of one thing: unrestrained egoism. Or, more simply: I want, and my wants are paramount. To hell with you. (Sometimes literally.)
Now, everyone enters the world as an unrestrained egoist. That’s all a newborn knows (ask any parent!): I want changing… now! I want feeding… now! I want comforting… now! But what happens --- or should happen, because lamentably, not all parents do their job properly --- is as soon as that newborn displays glimmerings of sentience, we spend as long as it takes trying to educate the egoism out of the kid with dictums no egoist wants to hear: you need to wait your turn. You need to play fair. Other people have wants and needs too, darling, and theirs are just as valid. We all have to play nicely with each other and get along. And most damning to the egoist: you’re not always going to get your way or what you want.
Unrestrained egoists --- villains --- never understand or accept this. They bulldoze their way through stories (and life) trampling anyone they feel is in the way of their desires. Again, why? Because that’s what villains/egoists do: I don’t care about you, or what you want. I want this, and I’m getting it. End of discussion. My advice is, don’t spend too much time agonizing over it. Yes, it’s awful. Yes, the world would be a better place if we played nice with everyone. But I think we have to be pragmatic and accept that, unfortunately, such desires have been around a very long time and don’t seem to be waning; on an immediate level, our task is simply to deal with them, anyway. That’s where our stories come in.
One of the really interesting things about evil is the idea of choice. This is basic to the whole discussion, absolutely basic. We all --- heroes and villains and ordinary folk alike --- choose our actions for good or ill (i.e. villains, by and large, choose to be villains). We are --- theoretically, anyway --- rational beings, capable of abstract thought, clearly knowing what we do and the consequences of our actions. If you want to get right down to it, the issue of choice as regards good and evil goes right back in literature to the Original Sin in the Garden of Eden. And still on a Biblical note, John Milton has Lucifer in Paradise Lost making his choice, saying as he is cast into the Pit that it is "better to reign in Hell than serve in Heaven." Now that's some choice! (But one made quite deliberately.)
Villains can be of two basic different temperaments --- purely evil, rejoicing, gloating and unrepentant in their awfulness (like Sauron in The Lord of the Rings, although he’s not exactly the most loquacious of characters), or essentially tragic figures who have fallen into evil ways, perhaps through poor choices and cannot bring themselves to return to good (due to pride, etc). LOTR’s Saruman is an example. That being said, my recommendation is to avoid making villains too one-dimensional. Sauron is Sauron, and we don’t necessarily have to know that he was bullied when he was little (just kidding, folks), but I think it helps if we can understand why villains are the way they are, at least to some extent. Plausibly, please. Because after all, even the most saintly among us experience egoist urgings. And we have an insatiable need to understand. Saruman is a perfect example, again. We understand his motivations --- and eventual regret --- much more in the book; while Christopher Lee was a great Saruman, I didn’t like what Peter Jackson did with the character. Film Saruman is far more one-dimensional, and while I know part of that was probably due to time constraints again (Saruman’s corruption of the Shire near book’s end is a hugely regrettable omission in the film), book Saruman is a much more sympathetic character, because Tolkien details why Saruman does what he does.
Strange, isn’t it? We don’t have to like villains --- shouldn’t, or they’re not really villains --- but boy, we sure want to understand them.