Lennie has trouble understanding his strength. Throughout the novel, the reader sees him accidentally kill animals, most notably, a puppy. Due to Lennie's disability, he views the world in a very child-like manner. He is not able to fully understand nor acknowledge why these animals died and that he was being too rough with them. His child-like manner of speech and behavior contrasts strongly with the violent actions that he takes. In a way, this shows that much of his characterization involves a gradual loss of innocence, ultimately coming to a head when he kills the puppy and hides it. This ultimately leads him to unintentionally killing Curley's wife.
[P1] Lennie can't comprehend his strength. [P2] Lennie's inability to comprehend his strength leads him to accidentally killing animals. [P3] Lennie's actions with the animals represent a loss of innocence that progresses throughout the novel. [P4] Lennie's last accident (with the puppy) leads him to the situation that ultimately causes his death.