After Curley's first confrontation with Lennie in the bunkhouse, George asks Candy why Curley became so hostile for no apparent reason. Candy replies:
"Well . . . tell you what. Curley's like a lot of little guys. He hates big guys. He's alla time picking scraps with big guys."
Obviously Curley suffers from an inferiority complex and does everything possible to hide it and overcompensate for it. When he first sees Lennie, who stands out because of his exceptional size, he clenches his fists and gives him a look described as "at once calculating and pugnacious." Lennie appears to be intimidated. He has been sternly warned by George not to get into any trouble.
Later when Curley comes to the bunkhouse looking for his wandering wife he gets into an argument with Slim, Carlson and Candy, who all laugh at him and ridicule him for not being able to control the flirtatious young girl. Ridicule is the things that brings out the worst in Curley because of his inferiority complex. He gets the erroneous idea that Lennie is also laughing at him and begins hitting him in the face. Finally George yellls, "Get him, Lennie. Don't let him do it."
With George's permission, Lennie fights back. He grabs one of Curley's hands and squeezes it until he breaks all the bones and joints, evidently crippling him for life. This will have bad future consequences. When Lennie accidentally kills Curley's wife in the barn, Curley already hates him and becomes homicidally enraged. He leads a mob after the fugitive with the intention of lynching him in the cruelest possible way.
The hostility that is exhibited by Curley, Slim, Candy, and Carlson exemplifies the competitive world George and Lennie are trying to escape from by owning their own little house and farm. Steinbeck portrays life for the poorest class as a constant struggle for mere survival. Ironically, these are the men who produce the food that everyone else needs.