As a general rule, it's important that your antagonist display some sympathetic characteristics.
It humanizes them, if you will (unless they're a vampire, demon or zombie). :-)
The reader will feel more than one way about the antagonist (beyond plain hatred), keeping them invested in the plot. It also helps the reader understand your antagonist's journey better.
Take Lord Voldemort in the HARRY POTTER books. When Dumbledore told Harry about Voldemort's childhood, it gave me a good sense of who Voldemort was and where he went wrong. I even felt sorry for him at one point, which made his evil deeds more gut-wrenching.
Movies and television shows create multi-dimensional antagonists all the time (if the script is well written). The masterpieces that were Godfather I and Godfather II made me care about and despise certain characters all at once, as did The Sopranos series.
Consider these antagonists and how we feel about them:
Damon in THE VAMPIRE DIARIES: He was the main antagonist at the very beginning of the series, but has evolved since then. Besides the fact that he's *bowchickawowwow*, Damon shows a softer side in his unrequited love for Elena, but continues killing innocent humans in fits of anger.
Sue Sylvester in GLEE: She a meanie and plots revenge, but still visits her developmentally disabled sister on the weekends (and calls her sister her hero).
Albie in BIG LOVE: Yes, he's a horribly mean, conniving killer. But, he was raised by a brutal father and is a closeted gay man yearning for love.
WHO ELSE CAN YOU ADD TO THIS LIST?
(photo sources: everyjoe.com, tvfantic.com, blogs.tampabay.com)
See You on Thursday!