Villains are interesting.
When you look at your prototypical Cambellian mythic hero, they're not really very complicated. While their temperament is variable, they always suffer to fight the good fight. Although they may struggle, their ultimate choice to is to stand against evil, turn back the darkness, and protect the innocent. So, their motivations aren't that complex.
A good villain on the other hand needs a really powerful motivation that is able to overcome the mores and ethics of their culture. The important thing to remember is, bad guys don't see themselves as villains. They don't commit crimes or hurt people to be "evil." If you want a strong, fascinating antagonist, there has to be a reason that they move and take action.
To me, those are the most interesting foils for heroes. Sometimes, the proper motivation can actually make you feel empathy for the antagonist, think Inspector Javert - born in prison, dedicating his life to law and justice to the point it blinds him to the inequality around him. So, we understand him, even if we can't accept his dogmatic pursuit of Valjean.
The proper motivation can make you despise a villain more while allowing you to better understand why they choose the actions they do. Look at Voldemort in Harry Potter -- he's just basically a racist. It's a reflection of the horrible things we see in our own culture - to worry more about purity of blood than worth of character. How many racist organizations are currently active in our enlightened age? Too many. But, Rowling embed him with a deeper motivation, his desire to be in control of his destiny, to be special, and his disgust of his origins ... this resonates with a lot of people.
I bring all this up as I begin to crystallize the antagonists for my new project. Last night, all the threads came together creating a pattern for my big bad's actions. It's really exciting when story points coalesce into something resembling a salient plot.
So, yay! I have a bad guy!