Is Curley's wife essential to "Of Mice and Men"?
Curley's wife is the final step in the acceleration of Lennie's struggle with himself. He starts out killing small mice because he is petting them to hard and both he and George feel that a puppy, being a little larger, might be a better fit for Lennie. We then find that that is too much for him when he kills the dog as well. Curley's wife is the final step and we see him killing her during the conflict with the petting of the hair. It seems here that Lennie is unable to control himself no matter how large or strong a character might be. If George did not kill him, Curley's wife would probably not be the last murder that Lennie would have committed.
Yes, she is! If she hadn't been there, Lennie wouldn't have accidentally strangled her. (This is the first crisis point of the story where an action can't be "undone.") He would not have had to "run for it," and George would not have been put into the difficult predicament of having to choose what to do before Curley's gang caught up with Lennie. If things had taken their natural course without these "domino" events, George and Lennie would have managed to buy their farm (along with Candy's participation) and fulfill their dream.