ARCAII'S VILLAINOUS METHODS OF VILLAINY
(Or, the basics of how to write a compelling and realistic villain)
I should note here that I haven't taken any classes or such about character creation - this is literally all I do in my spare time, so most of this was self-taught. I'm telling you now, in case some of what I say here is contradicted by people who actually do this for a job.
As well as that, this isn't going to be incredibly in-depth. This will just cover the most important parts of creating a villain.
There's no real set rules on character creation, just as there are no set rules for artists. Instead, there are suggestions and guidelines, which are optional to take, but generally recommended, because they often lead to a better character or art piece. However, there is one rule that I consider a complete necessity, and that is the HAT rule.
The HAT Rule, or Hero of Another Tale Rule, states that "every villain is the hero of their own story". Basically, your villain needs to firmly believe that what they're doing is right and just in order to be believable. That doesn't mean that you need to agree with what they're doing - you just need to understand why they're doing it, because this is the backbone of your villain's actions.
People are very much capable of convincing themselves they're doing the right thing, even if what they're doing is abhorrent. Take a look at how many people consider themselves morally correct for violently protesting against the existence of gay people, or how many slave-owners there were and are in the world. Your villain may believe in something terrible, but if they truly believe in it, then they will consider themselves a hero.
This, of course, requires a motive. The "why" is an important part of any character, but it's especially necessary for a villain. Without a motive and goal, your villain will fall flat.
I know what you're thinking. A motive to do evil things? A goal that those evil things are leading towards? A firm belief that they're in the right? That's all it takes to make up the basics of a good villain? Sounds pretty simple, doesn't it?
That's because, honestly, it is simple.
Writing a villain is not as hard as people make it out to be. It's essentially the same thing as writing a hero. That's why there are so many speeches about the hero and villain being "a lot alike" - they are a lot alike, at least in structure.
An analogy: Compare owning a Great Dane to owning a Chihuahua. People tend to treat these two creatures very differently - they'll let a Chihuahua jump all over them, bark at strangers and bite whoever they want, but they won't let a Great Dane do that. But why? They're the exact same animal, just a different size and shape. You wouldn't let a Great Dane jump on the mailman and bark at him, but with a Chihuahua it's seen as cute and harmless.
People assume that the Chihuahua is very different from a Great Dane, because they just seem so different. The truth is, there's no difference between them, and there's certainly no difference between training a big dog and a little dog.
Your hero and your villain are the same way. Both have goals and motives, and both firmly believe in what they're doing as right. Both will have character arcs, both will have backstories, both see themselves as heroes. The only difference is that your hero is considered a hero by the readers, and your villain is not. A Great Dane is considered a dog like any other breed, a Chihuahua isn't. Write your villain like you would write your hero, and you will find it to be much easier than you thought.
This is sort of a clunky post, but it gets the more important points across. Hopefully this helped someone. Let me know if something is unclear, and I'll do my best to answer it.