Why do you think George, who knows Lennie so well, tells him to “get him” [Curley]?

George knows that left unchecked, Curley will harass Lennie endlessly. When George gives him permission to fight back, Lennie cannot control his capacity for violence. He only stops crushing Curley's hand when George issues a direct order - leading one to wonder how he would behave in a similar situation is George were not there to control him.