Explain why do some people think Curley's wife is responsible for Lennie's death because of her background and her nature. John Steinbeck's "Of...

While nothing is directly stated about the background of Curley's wife other than that she is from Salinas, there are indications by her speech and her actions that she is of a low socio-economic background.  When she sneaks  into the stable where Lennie and Candy are sitting with Crooks, she does not hesitate to enter the Crooks's room.  This act is not one that a lady would take, especially because Crooks is black.  And, in the 1930s no "lady" crossed the racial line, let alone entered a man's room.  She speaks of how she could have been an actress by running off with an actor who came through town, but her mother would not allow her to leave. She tells Lennie, 

So I married Curley.  Met him out to the Riverside Dance Palace that same night.

In her behavior toward the men, as well as in her speech, she is brash, calling the men "bindle stiffs."  She tells them

If you had two bits in the worl', why you'd be in gettin' two shots of corn with and suckin' the bottom of the glass.  I know you guys.

Candy accuses her of having "floozy idears about what us guys amounts to," for she obviously has been around many men and frequented more than a few bars.  When she sees Lennie's face, she realizes that it is Lennie who has hurt the hand of her husband; she is impressed by Lennie's strength, and tells him,

I'm glad you bust up Curley a little bit.  He got it comin' to him.  Sometimes I'd like to bust him myself. 

After this incident, she comes around Lennie again while the others play horseshoes, desiring attention. When she encourages Lennie to stroke her hair, he becomes too rough and frightens her.  As she cries out, Lennie covers her mouth and tells her to be quiet lest he get into trouble with George.  When she struggles, her eyes "wild with terror," Lennie becomes angry, telling her to stop or she will get him in trouble, and shakes her, breaking her neck accidentally because he does not realize his own strength.

In his novella about the brotherhood of men, Curley's wife is sometimes regarded as a prototype of Eve, a seductress who comes around the bunkhouse and causes men to act in ways that they would not if left on their own.  She tempts Lennie with her hair, and the act becomes her final flirtation.  Still her motives may not have been sexual; she is extremely lonely and may have merely craved attention.