We could say that the main antagonist of the novel is the Great Depression; but as far as characters go, Curley is the main antagonist. Curley is a small man; perhaps because of his small stature, he compensates by fighting and trying to intimidate others. He represents a class above the other workers. Because he is the son of the boss, he can not be fired and he can have anyone fired. Therefore, he can provoke anyone he wants, fight anyone, and impose his will on anyone. The only person who stands up to him is Slim who seems to be revered by everyone. Curley does not have to face the same economic struggles that George, Lennie, and the other workers do. Curley has taken advantage of a young naive girl (his wife) because he can. He objectifies her and treats her as his possession. If Curley wins a fight, he can brag about it. If Curley loses to a bigger man, he can claim that the bigger man shouldn't have picked on him. The Swamper says:
S’pose Curley jumps a big guy an’ licks him. Ever’body says what a game guy Curley is. And s’pose he does the same thing and gets licked. Then ever’body says the big guy oughtta pick somebody his own size, and maybe they gang up on the big guy. Never did seem right to me. Seems like Curley ain’t givin’ nobody a chance.
Curley is in a position where he can not lose. He can not be fired and he can fire anyone. He comes out on top whether he loses or wins a fight. He is in a privileged position: one that he does not deserve. On the other hand, George and Lennie are in difficult positions, they struggle: whereas, they deserve better. This dichotomy illustrates an example of Naturalism in literature. George and Lennie lead difficult lives; as products of their social, natural, and economic environments. Curley is a human and symbolic representation of the social forces that oppress people like George and Lennie.