The antagonist of your story, depending on the genre in which you're writing, is just about as important as your protagonist. The antagonist is the bad guy, the opponent your hero will fight against and conquer. I mentioned genre because not all books contain a clear-cut villain.Gone with the Wind contains a lot of conflict, but no one antagonist. There are the Yankees who overrun the town, and they provide conflict, but they're also, for the most part, faceless representatives. The war itself, while providing external conflict, is really just a backdrop for the all the internal conflict Scarlett experiences and (sort of) overcomes.
Keep in mind also that "antagonist" and "villain" are not necessarily the same thing. An antagonist provides conflict and generates tension. Emmie Slattery, the impoverished "trash" in Gone with the Wind, is mentioned a few times and appears once in the book. She's an antagonist, certainly, but not necessarily a villain. So while all villains are antagonists, not all antagonists are villains.
But what about a story that does have a clear villain? In One Flew Over the Cuckoo's Nest, that would be Nurse Ratched, who dislikes everything about Randle McMurphy and in the end contributes largely to his destruction. But even Nurse Ratched, as 99% villainous as she is, is shown to have a human side (albeit briefly) in the book.
(And what about those names! Slattery sounds like "slatternly" and Ratched could be "wretched" -- both excellent descriptions of the characters.)
In the thriller The Silence of the Lambs, the villain of the piece is not Hannibal Lecter, as you might think, but rather the serial killer Buffalo Bill. And both of these despicable characters are given depth by showing at least one characteristic that humanizes them. With Hannibal Lecter, known as "Hannibal the Cannibal," it's his refinement - his appreciation of good food, music and manners; Buffalo Bill's soft side is shown in his love of his little white poodle, Precious.
So, just as the hero of your story isn't perfect - he must have his flaws to make him a well-rounded, believable character - your villain also must have at least some small redeeming qualities to keep him from becoming a caricature of the bad guy, a moustache-twirling Snidely Whiplash, so to speak.
"Keep in mind that the person to write for is yourself. Tell the story that you most desperately want to read." Susan Isaacs