The antagonist has good reason to grumble and scheme. The protagonist gets the star billing, but much of the story’s success depends on the bad guy.
The story role of antagonist brings conflict to the table, and conflict is what helps the story advance and stay interesting. Conflict, after all, makes the outcome uncertain.
It’s the antagonist’s job, therefore, to oppose the main character.
Unless you intend to create a one-dimensional, cartoonish villain, however, you need to figure out why the antagonist is in opposition. It should be a good, strong reason.
Now, beyond the basic responsibility of the antagonistic story role, we have something else to consider. Is your bad guy an antagonist or a villain?
Villains may be cruel, wicked, and devious. Antagonists may be annoying, obstructive, and stubborn. There’s a whole range of traits to choose from. Just make sure you design your character with ingenuity, determination, unpredictability, and some degree of ruthlessness. The degree of badness then depends on your plot and the genre you’re writing in.
For example, in a romance story the heroine plays the role of protagonist and the hero plays the role of antagonist. They have directly opposing goals: I want to marry him for happily ever after and bear his children VERSUS I am so not going to settle down, marry her, or have any kids! The plot deals with the conflict between the couple as physical attraction clashes with opposing goals. The hero, although the antagonist, is neither a terrible guy nor a villain. And once the pair’s goals become the same, the story ends happily for them both.
A thriller, by contrast, requires a villain. Someone who wants to do grievous harm–whether physically or psychologically–on someone else. Consider the novel HEART SICK by Chelsea Cain. The protagonist is a cop trying to catch a female serial killer but is instead captured by her and tortured gruesomely.
Bad guys are seductive to the writers that create them. After all, these characters can do or say just about anything. They don’t necessarily have a conscience to slow them down. It can prove liberating to write about them if you don’t try to hold them back or tame them too much.
If your antagonist starts to become a bolder, more vivid character than your hero, don’t squelch your villain! Go back and make a stronger character of your hero.