Curley is angry, vindictive, and violent. He's also the boss's son, which means he's got power—the power to make Lennie and George's life pretty miserable. As soon as they get even slightly comfortable on the ranch, Curley strolls in to make their lives miserable by picking on Lennie and not being able to control his flirty wife. In Steinbeck's book, that's totally his fault. In our book, that makes him the antagonist.