Curley's wife had previously dreamed of being an actress. Having married Curley, she resents the fact that she has to stay cooped in the house listening to Curley's bragging. She also resents being left alone while Curley is out working, playing horseshoes, or at a bar. So, she does go out and talk to the other men. She is purposefully flirtatious. There is a fine line between her loneliness and any flirting she does for affection or to get the men in trouble with Curley. That being said, one can't blame her for seeking to escape from Curley or for seeking companionship in what appears to be a lonely life on a ranch.
Curley's wife has no idea about Lennie's past history of unwitting violence and she certainly would not put herself in danger. So, there's no argument to suggest that she is responsible for her own death.
The responsibility belongs to Lennie and George. One could argue that Curley's neglect and poor treatment of his wife encouraged her to seek out companionship in the other ranchers; and this is what led to Curley's wife being with Lennie in the first place. But that is an indirect line of responsibility that goes along with the social structure of ranches in general. And that is to say that the ranch was not an ideal place for a woman and although Lennie was an incredible worker, he did not fit in socially; so it was not an ideal place for him either. Because of the situation, Lennie and Curley's wife find themselves in together in a place where neither of them really fit in. Lennie went with George because he had no one else. Curley's wife married Curley because she had nowhere else to go. In this frame, some of the responsibility lies in the unfortunate outcome of the social situation.
But as far as individual responsibility, Lennie and George share it. This is also unfortunate that the blame is theirs because both are in an awkward situation. Lennie kills Curley's wife; therefore, he is responsible. However, because of his mental handicap, he doesn't realize his own strength; and, when he panics, he simply doesn't know what he's doing. He is like a child and therefore, his guardian (or parent) should not have allowed him to be in such a situation where something like this might happen. George is that guardian and this makes him partially responsible for Curley's wife's death. He admits that he knew a tragedy like this might be a possibility:
"I should of knew," George said hopelessly. "I guess maybe way back in my head I did." (Chapter 5)
It seems unfair to blame George because he was faced with the task of watching Lennie, a grown man, 24 hours a day. Still, George and Lennie are directly responsible but it's easy to sympathize with them, understanding all aspects of the situation.