Curley’s wife’s death is a symbol of loss of innocence.
The death of Curley’s wife is a very sad time in indeed. She does nothing wrong, and Lennie kills her accidentally. Her death is therefore a tragedy, and symbolizes the end of Lennie’s carefree life. Although Lennie does not necessarily become a bad person after killing her, we can no longer think of him as a harmless man-child. He proves to be dangerous.
Her death also serves as the end of the innocent part of the story. Although things were not all roses before Curley’s wife dies, George and Lennie were moving along and doing fairly well. Their life was about survival, but centered around the innocent dream of the rabbits. After Curley’s wife is killed, there is no way George and Lennie can exist peacefully and continue their lifestyle. George has to put Lennie out of his misery.