Who killed Curley's wife in the novella Of Mice and Men?

Near the end of Steinbeck’s Of Mice and Men, Lennie kills Curley’s wife. Lennie did not mean to kill her. Lennie, despite his great size and strength, is meek and kind. But when he gets scared or threatened, he is unable to control his powerful strength.
At first, Lennie tries to stay away from Curley’s wife because George had warned him that she would cause trouble for them. But Curley’s wife is lonely, and she persuades Lennie to talk to her. First, she comforts Lennie over the death of his puppy. But really, she just wants a chance to talk about herself. Lennie is not too interested until she starts talking about her hair, which Lennie is in awe of because it is soft, and he loves soft things. She allows him to touch her hair, but as he is stroking her hair, he starts to get too rough. She tells him to stop, but he loves the feel of her hair and won’t. She panics and screams, resulting in Lennie’s own panic and his strong desire to keep her quiet. But in trying to muffle her, he doesn’t realize his strength, and he accidentally breaks her neck.
This event, as shocking as it is, is foreshadowed in the book. George, Lennie’s friend and protector, reveals to another ranch hand that they were forced to leave their last job because Lennie got into trouble when, wanting to reach for soft fabrics, he touched a woman's skirt. When she screamed, he panicked, causing him to hold on to her skirt even tighter until George had to hit him to loosen his grip. In addition, when George and Lennie are walking to their job, they discuss the mice Lennie used to have, given to him by his Aunt Clara. He loved to pet the mice but ultimately would kill them all because he was too rough with them. And finally, Lennie kills his own puppy by mistake. He loves his puppy so much, but when he was roughhousing with the puppy, he accidentally kills it.
None of these examples signaled any desire on Lennie’s part to harm another. And yet, the consequences of his final action, killing Curley’s wife, are dire. Even though Lennie’s actions are accidental, Curley, full of malice and the desire for revenge, will seek to torture and kill Lennie. George understands this as well, and so he runs after Lennie, to commit his final act of protection and love.

Towards the end of the novella, Lennie is lamenting over the death of his puppy when Curley's wife enters the barn and attempts to comfort him. Lennie tells Curley's wife that he enjoys petting soft things, and she allows him to touch her hair. Initially, Curley's wife enjoys the attention she receives from Lennie, but when Lennie begins to squeeze his hand, she starts screaming. Lennie panics and attempts to shut her up by tightening his grip. When Lennie shakes her head, he kills her by accidently snapping her neck. Lennie simply wanted to stop Curley's wife from screaming to avoid getting into trouble, but he was unaware of his own strength and murdered her. After killing Curley's wife, Lennie runs to his predetermined hiding spot away from the ranch and waits for George to meet him.