On the ranch, Slim is respected and Curley is reviled.
Slim and Curley are both men George and Lennie encounter on the ranch, and they are long-term residents. Slim is kind and well-respected, while Curley is regarded as trouble. Slim is mature, and Curley is immature. In terms of good and bad, it is easy to see how Slim and Curley are opposites.
The men trust Slim almost immediately, just as they distrust Curley. Both men have a position of authority, but Curley’s comes from his father owning the ranch. Slim’s comes from his strength and demeanor.
The way each man is introduced is indicative of their positions on the ranch. Slim is introduced as a good man.
Slim's a jerkline skinner. Hell of a nice fella. Slim don't need to wear no high-heeled boots on a grain team. (Ch. 2)
Candy’s comments about Curley are not as positive. He is spoken of in crude terms, both for his obsession over his wife and his tendency to fight. No one on the ranch seems to have any respect for Curley.
Curley's like a lot of little guys. He hates big guys. He's alla time picking scraps with big guys. Kind of like he's mad at 'em because he ain't a big guy. You seen little guys like that, ain't you? Always scrappy?" (Ch. 2)
The point Candy is making is that Curley overcompensates. He is mean and angry. When it comes to his wife, he is insanely jealous. One way or another, Curley is always spoiling for a fight.
The difference between the two men can be demonstrated in their reaction to Lennie. Curley is angry that George talks for Lennie. He is ready to pick a fight with Lennie because Lennie is bigger than he is. Slim, on the other hand, is impressed that George looks out for Lennie. He likes the fact that the two of them travel together, and he acknowledges that Lennie does not have to be intelligent to be a good person.
"He's a nice fella," said Slim. "Guy don't need no sense to be a nice fella. Seems to me sometimes it jus' works the other way around. Take a real smart guy and he ain't hardly ever a nice fella." (Ch. 3)
On the ranch, most people only look out for themselves. Slim is one of the few men who can actually make friends. Curley alienates everyone around him by fighting or accusing them.