Villains. Every good story has a hero, and every good story has an antagonist - be it nature, inner conflict, a supernatural force, a person, or all of the above. But the largely debated question is: what makes a good villain? And honestly, I don't believe that question will ever fully be answered. But, in this next wall of text, I'll list off the many villain types, both good and bad. But first off, before we officially begin, each type of story has its own type of villain. If you're looking to include all of the types then you're going to have a hell of a time trying to manage it, because not all of them fit well in a single story.
1. The Purely Evil Villain. If you've ever read The Lord of the Rings then you know the conflict is between the forces of Good and the forces of Evil. The Evil forces are represented by Sauron, who may very well be the perfect example of an all evil villain. Evil villains are motivated by their need to cause chaos and destruction to the forces of Good, which is normally the hero(s). They very rarely have any other motivations for their actions besides the fact that they are, essentially, evil. 'Demons', such as in horror novels, are other examples of purely Evil villains. In the movie The Poltergeist, Reverand Kane is another example. Purely Evil villains normally work best in a story setting where there is a definate Good force. Purely Evil Villains are a good way to get your readers completely allied with the hero.
*The Power-Hungry Villain. I included this part-way in the Purely Evil Villain description because it in itself isn't a completely valid reason. Power-Hungry Villains, as their name implies, are driven by their need to overpower and control others. Sometimes this includes a detailed backstory on why they feel the overwhelming need to control others, but for the most part (again, with Sauron,) they wish to become powerful to spread more chaos and destruction, thus being ruled as completely 'Evil'. Another example of a Power-Hungry Villain is Ellsworth M. Toohey from Ayn Rand's 'The Fountainhead'.
2. The Jealous Villain. Many villains, especially in romance-oriented stories, are fueled by jealousy, mainly towards the hero's abilities and/or possessions/girlfriend/wife. Jealous Villains are typically very shallow, being their only motivation is the want to be like the hero, or have what the hero has. These are generally personal issues, and the villains often see the hero 'head-to-head'. Very seldom do you see Jealous Villains in Epics like The Lord of the Rings or Star Wars.
3. The Backstory Villain. One of the two most complex types of villains, the Backstory Villain has the most numerous options of why he (or she, not trying to be sexist here) is evil. It can be anything as simple as a bully beat him up in Kindergarden or as horrific as he was severly mutilated or his parents abused him. For the most part Backstory Villains had a very traumatic experience as a child/young adult that caused them to want to seek revenge, take it out on the world, etc. To use a real life example, Hitler (though possibly partially insane) was abused as a child and had his mother die at a young age. Seeing what he did later on in life provides a good example of what a Backstory Villain is capable of.
4. The Insane Villain. My most hated type of villain, Insane Villains are evil almost parallel to how Purely Evil villains are evil. The only exception is that they suffer from a natural 'mental disease' that causes them to commit the acts they do. Insane villains are the most cliche and overused type of villains, as they're the easiest to write about. But why are they the easiest to write about? It's because they can do anything at any time, without any logical reason whatsoever. If Bob the Villain wants to bomb a building, gun down a bunch of civilians and then fly to Japan and do the same thing with no reason on why he did he can, because he's insane. 'Insanity' has become the 'Get Out of Jail Free Card' for many writers this day and age, inspired by the most overrated villain of all time: The Joker. This is a villain type you want to avoid. However, though I'm not trying to encourage the making of an Insane Villain, it can be done right. If you've ever read Dean Koontz's 'Intensity', it has a nearly insane villain that has very believable and logical actions. That's the only Insane Villain I've seen done right, though.
5. The Love Villain. The other most complex and almost a backstory villain, The Love Villain deserves a title to istelf because its actions can depend upon a multitude of things. Love Villains, typically, act the way they do because they either weren't shown love as a child or their lover died, left them, etc. For example, most of the reason Darth Vader turned into a Sith was because he wanted to become more powerful so he could bring his dead mother back. Because of this, Love Villains are possibly the most reader-sympathetic out of the villain types, because love is (hopefully) something we can all identify with. Love is also a good way to have your hero turn evil, if that's the direction your story is going.
No matter which villain type suits your needs, your villain MUST have a reason for being the villain. If he doesn't then he's considered flat, and a flat villain means a flat plot. ALso, you don't have to limit yourself to just one villain; there can be multiple main villains, especially if you're writing a series. Multiple villains creates multiple conflicts which can enchance your story if you do it right.
Now for just some general villain tips. First off, your villain's appearance says alot about him and the time period in which he lives. You aren't going to stick blue jeans and a T-shirt on a villain in Midevil Times, and normally you won't clad a villain in armor in modern-day New York, unless he has a damn good reason. And that's another thing: as awesome as it looks to give your villain a badass mask, unless he just absolutely needs said mask then he shouldn't wear it. For example my villain (who is modeled after Sturm from the game 'Advance Wars') wears a gas mask not only because it looks cool, but also because he needs to breathe pure oxygen in order to survive. But also remember the eviler a villain appears the more credible he will seem to your readers. Not saying all villains should be dressed in black hooded robes and masks, but it gives off that 'creepy-villain' feeling. Again, if you've ever read 'The Lord of the Rings', the Ring-Wraiths are good examples of fear-inducing villains, both to the hero and the reader.
Well, that about does it for this guide. Hope this helps anybody; comments are highly reccommended.