Curley is presented in Of Mice and Men in a very negative way. Let me name three ways this is developed.
First, Curley does not have the respect of the other characters. Whenever Curley is brought up in the story, he is mocked or belittled. This is clearly seen in his interaction with the men. Even Carlson, a relatively minor figure, tells off Curley. Here is what text says:
Carlson laughed. “You God damn punk,” he said. “You tried to throw a scare into Slim, an’ you couldn’t make it stick. Slim throwed a scare into you. You’re yella as a frog belly. I don’t care if you’re the best welter in the country. You come for me, an’ I’ll kick your God damn head off.”
Second, Curley's wife does not love him. What makes this even more powerful is that they only have been married a few weeks. The men say that Curley's wife has the eye. What they mean by this is that she has the eye for other men. This decidedly casts Curley in a negative light.
Finally, Curley shows his insecurity and brutality in his picking a fight with Lennie, an innocent man. Curley begins to punch Lennie mercilessly. Lennie is a bloody mess. But the irony is that when Lennie begins to fight back, he defeats Curley with one hand. This episode further diminishes Curley.