The predatory nature of man is represented in the men's relationships with one another.
To begin, when George and Lennie arrive late to their ranch destination, the boss' frustration with their tardiness is shared by not just one, but a few characters.
As they learn about the boss, Candy reveals that the boss uses Crooks, the black stable buck, as a punching bag. Crooks bears the brunt of the predatory nature of the boss.
Curley is learning from his father. Although Curley is a little man, he has no problem making the other characters feel threatened by him. His purpose is to scare anyone away from his new wife as if the other men would have the nerve to hit on her. Curley feels particularly threatening toward Lennie. He shows this need to protect himself by lashing out on Lennie. He begins attacking Lennie like a predator attacks his prey, but Lennie has an obvious size and strength advantage.
Lennie could be a predator of other men, but his heart does not desire to do that. He even waits for the command from George to protect himself.
Once Lennie commits his final act of accidental death, Curley's predatory instinct kicks in again and seeks justice for his deceased wife, as well as revenge for the loss that Lennie caused him.