Even people who would do just about anything to avoid conflict in their own homes or at their jobs often want a real troublemaker in a story they read or a show they watch. Most every protagonist needs somebody to pit will or brains or beauty against. A story can have one or more antagonists, but almost all require this role to be filled in order to create narrative tension that will keep the reader reading, the story unfolding, and somebody for both the reader and the protagonist to want to beat.
The antagonist actually has a widely varying and extremely important job. In good literature and the finest films, this character might be hard to spot, at least initially. The main character, or protagonist, might think this anti-version is really a best friend or true love. It is usually the reader or viewer who gets it first and spends the rest of the story waiting for the protagonist to recognize the bad guy.
A good storyteller, regardless of medium, invests an antagonist with at least some saving graces. Perhaps this individual was deeply wounded as a child, or perhaps he or she truly believes in a moral righteousness that the audience can see is misguided. Some of these characters begin on the protagonist’s side and are driven or seduced away by circumstances beyond their control.
A well-crafted antagonist helps the plot to unfold by tugging the narrative thread just enough to force the story forward. Often, an author gives the villain the opportunity to drive the story into new and unexpected twists and turns. Those that exhibit round or deeply human feelings, motivations, and needs are often easier to forgive, and they are also more likely to spin twists into the action that are surprising.
Genre fiction and films, such as works that deal with magic, murder, space creatures, and the like, are often less likely to have highly developed, complex antagonists. These types of stories are more likely to depend upon flatter characters with stock behaviors that are easy for the audience to recognize. The witch in the fairy tale, the troll in the folktale, and the insane murderer in the slasher film are examples of the types of bad guys that audiences love to hate and have no need to humanize.