Curley's wife is the person who discovers Lennie alone with the dead puppy; she frightens Lennie and in an effort to keep her from yelling out loud and notifying others of what he has done, he attempts to keep her quiet. In doing so, he ends up killing her in the same way he did the puppy.
In the fifth section, she discovers Lennie's dead puppy in the hay.
Curley's wife discovers Lennie's puppy is dead.
Lennie kept on trying to hide it because he was afraid he was going to get scolded by Curley's wife or she was going to tell on him.
Lennie was told multiple times to leave the puppy alone so it won't die but Lennie kept on disobeying him.
She discovers a dead puppy that Lennie has accidentally killed. She is frightened and Lennie does not want people to find out so to stop her screaming he breaks her neck. He then runs away like George said to do if he ever got in trouble.