The Antagonist, or, Why Most Bad Guys Suck.
What is a hero without a good villain?
BORING.
This is the problem I have with most bad guys outside of comedy and parody stories. Unless you're Sauron or The Joker, being evil just because isn't enough. (And sometimes even then...) Good villains -- antagonists with personality, motive, and a human side -- can save a story. They can make a boring hero seem interesting (This is why Batman is awesome and Superman is not; Batsy's villains give him a challenge occasionally and have personality. Supe's rogue's gallery? Not so much).
The best antagonists are ones you could almost agree with for their actions. Except... Those two words, "almost" and "except", are important. Lupin III would be an awesome guy to hang with, except for his womanizing and thievery. One can almost relate to Mr. Freeze's desire to find a cure for his wife's affliction if one ignored his terrorizing Gotham to do so.
A good antagonist can and should have some parallels to the hero. As the Joker said in The Killing Joke, all it takes is one bad day. A hero should, once he gets to know the villiain, look at him and say "There but for the grace of God go I". This means that the villian should have at least one redeeming trait. Again, look to Freeze's love and care of his wife. (This also means the hero should have at least one trait that makes him annoying at best. Think of Dr. Gregory House's arrogance and misanthropy. But that's a rant for another day.)
Most importantly, a good antagonist has a PLAUSIBLE, RELATABLE or at the very least INTERESTING MOTIVE for their villainous actions. In The Dark Knight, the Joker's deeds all seemed random. But upon closer inspection I found a common thread. Each act made people question their own behavior. Joker's motive is his belief that modern society's laws and beliefs and accepted behaviors are a joke. And he intends to make everyone get the punchline even if it kills them. ("I'm an agent of chaos. And you know the thing about chaos? It's fair.")
Making a good villain is tougher than making a good hero. Besides what I listed, it helps if the antagonist is at least as intelligent as the hero. (Dr. Draken made this point in his plot from the movie "Kim Possible: So The Drama". If his sidekick Shego, who is as smart as protagonist Kim, couldn't figure out his scheme, neither would Kim. Presumably.) He should have something he fears, even if it's just his plot being found out. He should have flaws and limitations just as great as his powers and abilities. And if one is writing a series, every so often, the bad guy should win.
These are things I ave to keep in mind as I work on the story.