Why does Curley keep quiet about what Lennie did to him and not try to get George and Lennie fired? How does this reflect on Curley's character?

Lennie smashed Curley's hand. Curley kept quiet because he was too prideful to let anyone know he'd been beaten. That, and Slim told him that if he said anything and Lennie got fired.... they'd make sure he was a laughingstock.

Curley nodded. "Well, then listen," Slim went on. "I think you got your han' caught in a machine. If you don't tell nobody what happened, we ain't going to. But you jus' tell an' try to get this guy canned and we'll tell ever'body, an' then will you get the laugh."