What is the role of Curley in Of Mice and Men?

Curley is a classic antagonist to the protagonists George and Lennie in Steinbeck's novella Of Mice and Men. The antagonist in a piece of fiction is a character who stands against the main character or characters. From the very first time George and Lennie meet Curley in the bunkhouse in chapter two, they consider him a threat because of his tough-guy attitude. In their first conversation, Curley insists that Lennie talk, hoping from the very start that he can provoke a fight with the big man. Candy explains it best when he claims that Curley likes to pick on big guys such as Lennie:

Well...tell you what. Curley's like a lot a little guys. He hates big guys. He's alla time picking scraps with big guys. Kind of like he's mad at 'em because he ain't a big guy. You seen little guys like that, ain't you? Always scrappy?

After meeting Curley and then Curley's wife, Lennie's childlike instincts tell him that the ranch is not the right place for the two men. He pleads with George "Le's go, George. Le's get outta here. It's mean here." Lennie foreshadows the problems which will arise because of Curley's "pugnacious" attitude and the seductive qualities of Curley's wife. George acknowledges Lennie's fears but is determined to earn money to purchase his dream ranch. But he too worries about eventually fighting Curley, saying, "Ya know Lennie, I'm scared I'm gonna tangle with that bastard myself. I hate his guts."
In chapter three, Curley is rumored to be looking to fight Slim because he believes the skinner might be in the barn with his wife. Apparently this scene is typical on the ranch, due to Curley's constant jealousy toward his young wife. While this is going on, George once again describes the longed for ranch, this time exciting the aspirations of old Candy, who pledges to donate money to the project. While George, Lennie, and Candy muse over the future, Curley and the other men burst into the bunkhouse and Curley instantly lights into Lennie, punching him several times before Lennie fights back and subdues Curley by catching the smaller man's fist in midair:

Curley's fist was swinging when Lennie reached for it. The next minute Curley was flopping like a fish on a line, and his closed fist was lost in Lennie's big hand.

George ultimately gets Lennie to let go of Curley's hand, but not before the hand is badly crushed by Lennie's overpowering strength. Afterward, Curley is subdued, agreeing not to tell anyone what happened to his hand so that George and Lennie can continue working on the ranch. Unfortunately, Lennie's beating of Curley arouses the admiration of Curley's wife, who quizzes Lennie on how he got "them bruises" on his face. This unintended attention comes to a climax in chapter five when Curley's wife insists that Lennie sit and talk with her. Soon enough, Lennie is stroking her hair, leading to his grabbing and shaking her. Steinbeck uses familiar language to describe Lennie's throttling of the girl:

He shook her then, and he was angry with her. "Don't you go yellin," he said, and he shook her; and her body flopped like a fish. And then she was still, for Lennie had broken her neck.

After this incident, Curley's behavior is predictable. He immediately wants to kill Lennie, who has escaped to the "brush" by the Salinas River (which was the setting of the first chapter). Curley seems oblivious to reason and sets off armed with a shotgun. The reality that Curley will shoot Lennie, or, at the very least, the big man will be locked up in jail, prompts George to kill Lennie himself. Rather than seeing his friend suffer, George shoots Lennie in the back of the head and the man is dead instantly. At the end of the novel, Curley shows a glimmer of humanity when he examines Lennie's body: "'Right in the back of the head,' he said softly."
While readers may blame Curley and his wife for the tragedy of Lennie's death, it seems likely from evidence early in the novel that George would have ended up in the same position no matter where the men had traveled.