Curley(owner's son) feels intimidated by Lennie because Lennie is bigger. So, Curley picks a fight with Lennie resulting in Curley's broken hand.
George got Lennie a puppy that he kills accidentally due to his strength .
Curley's wife comes into the barn where Lennie is with the dead dog and she tells him about how she could have been an actress but ended up marrying Curley instead.
Lennie tells Curley's wife about how he likes petting soft things. So, she tells him that her hair is soft and lets him pet it. But, he starts petting her hair too forcefully and after a struggle he ends up breaking her neck.
Everyone on the farm figures out that Lennie killed Curley's wife they go out to look for him.
George shoots Lennie in the back of the head because He would rather have Lennie die by his hands than the hands of Curley and the others.