In Of Mice and Men, is George the one really responsible for the death of Curley's wife?
George represents a parent to Lennie, but Lennie believes George is something akin to God, all-knowing and all-seeing. This is a huge ego boost to George, who is intelligent, but hardly omnipresent.
This is an interesting question and is not easily answered. George does take responsibility for Lennie when Lennie's caretaker, Aunt Clara, dies. Since Lennie is like a child, George is essentially his parent and therefore George is responsible for what Lennie does. However, Lennie is a child in the body of a powerful man; a well-meaning but difficult man-child to take care of. And being a man, Lennie is forced into the life of a migrant rancher, one he is more than physically equipped for, but one that he is not mentally equipped for. So, George is responsible to the extent that he is Lennie's parental figure but it is not George's fault that the circumstances of Lennie's mental handicap and the harsh life he (and George) are forced into often combine to disastrous ends.
If you look at the complexity of the situation, Curley's wife's death is the result of multiple actions taken by multiple people, and her death then can be seen as a socio-economic effect. Curley's wife, a once aspiring actress, has settled for Curley. Curley is an insecure laborer who can provide for her financially but who treats her poorly. Curley's wife, like Lennie, is forced to live on a ranch (hardly an ideal place for a woman in those times), so she actively engages the other men not just to flirt but out of a genuine need for companionship. This exacerbates Curley and makes some of the men, namely Lennie, uncomfortable. And when Lennie is uncomfortable he acts out, sometimes violently.
Because Lennie had nowhere else to go, he ends up at the ranch; the same is true for, for Curley's wife. Curley's wife's career never took off (for whatever reason) so she needed someone: Curley. Likewise, there was no place in society for Lennie (sans jail or a mental ward) so he ends up at the ranch. So, there are social and economic issues (which is the fault of society as a whole) that lead to Curley's wife and Lennie even being at the ranch in the first place.
That being said, George accepts responsibility for what Lennie does but one can sympathize with George's plight. You could argue that Lennie's admiration gives George some pride but George gets more of a companion in Lennie than he does an ego boost. And in this sense, Lennie is his friend as much as he is like his child. Curley's wife's death can be attributed to a number of things and we can blame George for not having been there to supervise Lennie every minute of every day, but that is a harsh criticism. No one can expect George to be omnipresent/omniscient.
George kills Lennie to save him from Curley and to ensure that Lennie won't be able to hurt anyone again. Slim recognizes that George had to do this:
Slim came directly to George and sat down beside him, sat very close to him. "Never you mind," said Slim. "A guy got to sometimes."
This is a telling moment. Lennie had nowhere to go, so George took him in. In the end, the only way to prevent Lennie from inevitably getting into (often violent) trouble was to kill him. So, George had always been faced with an impossible decision (getting rid of Lennie or managing Lennie's behavior as best as he can, which proved to be an impossible task for one person).