In plotting his book, John Steinbeck had a complex and sensitive problem with the character of Curley's wife. It was inevitable that she would be killed by Lennie so that he would have to run away and later be killed by his friend George. That was the essence of the story. Naturally the reader would feel a certain amount of sympathy for a pretty girl whose life is snuffed out so abruptly and needlessly. But the author did not want to undercut the sympathy the reader would feel for Lennie when he gets killed in the last chapter by his trusted friend. Steinbeck uses the fourth chapter in part to show Curley's wife in a bad light, so that she will not seem to be nothing more than an innocent young victim of a brutal murder when it happens.
Curley's wife is trouble. She intrudes into Crooks's room and refuses to leave. She behaves flirtatiously, creating the impression that she is promiscuous and immoral and that she doesn't care about what trouble she causes for Lennie, Candy or Crooks.
Curley's wife is vicious. She attacks Crooks with abusive language. At one point she says:
"Well, you keep your place then, nigger. I could get you strung up on a tree so easy it ain't even funny."
The author's intention is obviously to make the girl look bad--but Lennie's crime has to look bad too; otherwise George would not seem justified in shooting him. So after Curley's wife is dead in the next chapter, Steinbeck softens her image with the following description:
Curley's wife lay with a half-covering of yellow hay. And the meanness and the plannings and the discontent and the ache for attention were all gone from her face. She was very pretty and simple, and her face was sweet and young.
As readers we end up feeling sorry for three characters--Curley's wife, Lennie, and George. What made Steinbeck an important writer was that he had more than just talent; he had a heart filled with sympathy for all humanity, and especially the humble and oppressed.