The character of Crooks is the practical stereotype of black people in Depression-era America: proud, bitter and bitingly sarcastic. He is opportunistic, taking advantage of Lennie’s mental disabilities to belittle him without taking his physical strength into account. Crooks is cynical and cold-natured because this is how he has been treated all his life. He is also intelligent and capable of reading, which was a rare skill for black people at the time.
The main reason behind his cruel behaviour is that he is very lonely. He is the only black man on the ranch and he experiences a lot of racism and discrimination. He has no one to talk to and he feels lonely, which leads him to abuse those who are weaker than himself. This explains why he torments Lennie by suggesting that George will abandon him. He takes pleasure in mentally hurting Lennie, but only because he is so lonely.
While the other men share a bunkhouse, Crooks sleeps alone in his stable. He is a stable buck, which means that he looks after the horses and is sort of like an on-site vet and groom. He is a vital member of the crew, but no-one shows him any respect. Candy even refers to him as ‘the nigger’ or ‘the stable buck’, which shows how little status he has. He is only recognised for his colour and his job, not as a person.