I love a good antagonist.
One point that really stuck out for me was:
4. Make sure your antagonist—at least in his own mind—has justification for everything he does. He has to believe he’s in the right, otherwise your reader won’t feel he’s real and will disconnect. To make him believable, be sure he believes in himself and every step he takes.
Because let’s face it, no one believes they are evil. We all believe in our own minds we are the right ones. We are doing what is best for ourselves and sometimes others. Our minds are amazing devices that ensure we think the best of ourselves, despite doubts and ‘issues’, we still play the hero in our own stories. For whatever reason your antagonist has to feel justified for his or her beliefs and methods. Even if we never directly hear from them. We need to know what those justifications are to write so we do not lose the reader.
It is a good idea to think well and long about our antagonist. What makes them tick. What motivates them.
There must also be a sort of balance of power. We cannot have them be too powerful. Nor too weak. It seems best they are of equal strength to our protagonist if not a little stronger.
Here is what one of my favorite authors Jim Butcher has to say about ‘How to build a villain’
1) Motivation. Your villain has to be motivated even more strongly than your protagonist, to move in a direction that is opposite to your protagonist’s goal. The drama and tension of the entire story is based upon those two opposing forces. Buffy versus vampires. Sith versus Jedi. Spy versus spy.
2) Power. Your villain has to have enough power, of whatever nature, at his disposal to make him a credible threat to your hero. Personally, I believe that the more the villain outclasses the hero, the better. David wouldn’t have gotten nearly the press he did if Goliath had been 5’9” and asthmatic.
3) Admirable Qualities. Every serious “big bad villain” you write ought to have facets of his personality that are desirable, even admirable. Perhaps your villain is exquisitely polite and courteous, extremely perceptive, remarkably intelligent, or possessed of a skewed sense of honor that makes him something more than a simple black-hat. In point of fact, a villain might be loaded down with admirable qualities, all of which should serve to only make him even more dangerous to your protagonist. Think of the Mayor of Sunnydale in “Buffy the Vampire Slayer.” Except for the part where he was trying to turn himself into a giant demon and devour the graduating class, he was a great guy!
4) Individuality. A good villain needs to be instantly recognizable to your reader, so that even if he hasn’t appeared in a hundred pages, your reader will recognize that character instantly. You can achieve this pretty effectively using Tags and Traits, identifiers for a character which reserve particular props, personality traits, and words to associate with any given character. You can find an article that goes into them in greater depth on my livejournal at jimbutcher.livejournal.com.
You can have people sympathize with the antagonist. They can have some damn good characteristics we can relate to. There can be a powerful story behind why they are the way they are. Even some strong points of why they are doing what they are doing. And then it is good to have a distinction tossed in between your protagonist and antagonist to throw people off. So they Know why the antagonist is the antagonist.
“In Blake Snyder’s books, he speaks of giving the hero a “Save the Cat” moment — meaning, we get to rally behind the protagonist early on as we get to see just what he’s capable of because, y’know, he rescues the cat from the tree (metaphorically). Antagonists need the reverse: one requires a “Kick the Cat” moment (see also: “Detonate the Puppy,” “Machine Gun the Dolphin,” or “Force the Baby Seal to Watch a Marathon of the Real Houswives …). We need to see just why the antagonist is the antagonist — show us an act that reveals for us the depths of his trouble-making, his hatred, his perversion of the ethical laws and social mores of man.” Chuck Wendeg 25 things you should know about antagonists
In the end you want to write that antagonist people love to hate. Or even hate to hate.