Slim ends up giving Lennie one his puppies, and George warns him about petting it too rough and taking it out of its nest. In Chapter 5, Lennie accidently kills his puppy by being too rough with it. When Lennie is holding the dead puppy, he looks at it and says, "You ain't so little as mice. I didn't bounce you hard" (Steinbeck 42). Interestingly, Lennie is not upset at himself. He is upset at the puppy for dying. He does not take responsibility for being too rough with the puppy and begins to worry about George's reaction. He fears that George will not allow him to tend rabbits and contemplates hiding the dead puppy. Lennie then erupts and begins yelling at the dead puppy and throws it across the room. Lennie's reaction demonstrates his lack of perspective and understanding. Instead of realizing that he was at fault, Lennie blames the puppy. The only consequence that Lennie fears is George's reaction to finding out he killed the puppy. Lennie is obsessed with his dream of tending rabbits and is worried that he will never have the opportunity to take care of them.

Lennie accidentally kills his puppy, probably by squeezing him or hitting him too hard. Although Lennie is upset at the death of his puppy, he is more concerned about George's reaction. He is afraid that when George sees he killed his puppy, George will not let him tend the rabbits on the farm they have dreamed about owning. So, he tries to hide the body of the puppy only to have it discovered by Curley's wife. This leads to further disaster. Lennie's has been told to stay away from Curley's wife but, after a conversation with her, he accidentally kills her as well. This event forces George to have to make a final decision about what to do about Lennie. Curley's wife is not an animal and this escalation of Lennie's uncontrollable control over his own strength means Lennie faces execution or long prison term.

Lennie "loves his puppy to death". Not knowing his own strength, Lennie was too rough with his puppy and ended up killing it. Like the dead mouse he had at the beginning, Lennie continues to stroke it because it's soft. Lennie's biggest fear at this point, though, is George's reaction. Lennie is afraid that since he killed the puppy, George will not allow him to take care of the rabbits when they get their ranch.