If one of the themes in Of Mice and Men is loneliness and exclusion, another could be “man’s inhumanity to man” or the persistence of social hierarchies, even in the constrained circumstances of the ranch.
Crooks, as a Black man, has been forced out of the bunkhouse by the other men and sleeps by himself. Lenny, however, is someone of an even lower social order than his; this is why he torments Lenny, suggesting that George might have “taken a powder” and left Lenny by himself. When Lenny finally understands what Crooks is suggesting, he is barely able to contain his fury; Crooks backs down, and says he was only trying to get Lenny to think about what it might be like to be alone.
The episode underlines the fundamental problem of the book. What Crooks craves is someone he can explain his loneliness to; Lenny is at best an imperfect partner for this, but Crooks’ desire for companionship is every bit as fanciful as Lenny’s certainty about raising rabbits with George. Crooks recognizes the impossibility of realizing their dreams. He says to Lenny
I seen hunderds of men come by on the road an’ on the ranches, with their bindles on their back an’ that same damn thing in their heads. Hunderds of them. They come, an’ they quit an’ go on; an’ every damn one of ‘em’s got a little piece of land in his head. An’ never a God damn one of ‘em ever gets it. Just like heaven. Ever’body wants a little piece of lan’. I read plenty of books out here. Nobody never gets to heaven, and nobody gets no land. It’s just in their head.
Crooks bitterness is not shared by Lenny, who has complete faith in George and in the truth of his promise about the rabbits. Perhaps the reason for his tormenting Lenny is that, despite his lack of intelligence, he has something Crooks lacks: someone who cares for him, and a dream of a better life.
In chapter four, George has gone into Soledad on a Saturday night with some of the men, leaving Lennie behind. While Lennie is in the barn playing with his puppy, he sees the light from Crooks's room and hesitantly comes to the doorway. Crooks is not at first happy about the intrusion but eventually allows Lennie into his room. He explains to Lennie about how he is not allowed in the bunkhouse because he's black and the men say he "stinks." In an act of revenge against his segregation and loneliness, Crooks begins to torment Lennie. He tells the big man that maybe George won't ever come back and Lennie will be left alone:
“S’pose George don’t come back no more. S’pose he took a powder and just ain’t coming back. What’ll you do then?”
Crooks seems to take great pleasure in torturing Lennie in his suggestion that Lennie would wind up in the "booby hatch" if George were not around to take care of him. He has won a "private victory" in his taunting of Lennie. But, as soon as Lennie begins to understand the gravity of Crooks's words, he works himself into a rage:
Suddenly Lennie’s eyes centered and grew quiet, and mad. He stood up and walked dangerously toward Crooks. “Who hurt George?” he demanded.
Obviously Lennie has a temper which he is often unable to control. Realizing that he may have unleashed an unknown power, Crooks quickly diffuses the situation and insists that he was only "supposin'" and that George is not hurt. He then explains to Lennie how it feels to be alone all the time and that Lennie is lucky to have George:
“Maybe you can see now. You got George. You know he’s goin’ to come back. S’pose you didn’t have nobody. S’pose you couldn’t go into the bunk house and play rummy ‘cause you was black. How’d you like that? S’pose you had to sit out here an’ read books. Sure you could play horseshoes till it got dark, but then you got to read books. Books ain’t no good. A guy needs somebody—to be near him.”
Crooks's confession about his life serves to strengthen Steinbeck's theme of loneliness which pervades this chapter and the entire book.
Crooks suggests that something might happen to George, to prevent them from getting their farm (and dream...and the rabbits!).
Lennie starts to get mad, which is very dangerous given his strength.
Crooks does this because the isolation he suffers due to his race and the treatment given him for it from the men has made him bitter (the treatment and his spine). He's getting a little revenge.