Steinbeck's story is heavily plotted although it is intended to appear realistic or naturalistic. Steinbeck's plot problems and solutions can be inferred from the text. Evidently his main purpose was to dramatize the hard lives of the kind of men who did the heavy labor on big ranches. He knew such men personally. He was a democratic, convivial man whose sympathy and compassion for such men is obvious in all his writing. He was not a romanticist or idealist, however; he knew that there is good and bad in everyone.
Steinbeck wanted a short novel because he intended to adapt it to a stage play, as explained in eNotes Study Guide Introduction. He also wanted to end his story with a man killing his best friend as an act of mercy. Lennie would have to do something exceptionally bad in order to merit killing. Steinbeck foreshadows Lennie's partially accidental killing of Curley's wife by having George twice relate--once to Lennie himself and again to Slim--what he knows about the incident that occurred in Weed.
Curley's wife's death leads directly to Lennie's execution, and both are tied to the incident in Weed, which neither George nor the reader understands completely until something similar takes place in the barn on the ranch south of Salinas. When George sees Curley's wife's body in the stable, he decides that he can no longer act as Lennie's guardian. Lennie is becoming dangerous and possibly uncontrollable.
Curley's wife lay with a half-covering of yellow hay. And the meanness and the plannings and the discontent and the ache for attention were all gone from her face. She was very pretty and simple, and her face was sweet and young. Now her rouged cheeks and her reddened lips made heer seem alive and sleeping very lightly. The curls, tiny little sausages, were spread on the hay behind her head, and her lips were parted.
What happened in the barn could have happened in Weed, too, if the girl hadn't screamed and attracted George and other men. And it could happen again and again if George helped Lennie to escape into the Gabilan mountains and fled with him to some other part of California.
"I should have knew," George said hopelessly. "I guess maybe way back in my head I did."
George did not see what happened in the barn. What it looks like is attempted rape leading to murder to keep the girl from screaming-- which could have been close to the truth. George is thinking about Weed. He didn't see what happened there, either. He and Lennie had to start running, so the only explanation he got was from Lennie, and Lennie consistently tells him lies. George is thinking that Lennie was not interested in feeling the girl's dress but was molesting her, possibly even attacking her, on the main street in broad daylight. Lennie's interest in petting soft little mice, puppies and rabbits is evolving into a sexual interest in soft, pretty young girls which Lennie is too mentally retarded to be able to understand or control.
The reader understands the Weed incident in a new light at the same time as George and also understands why it is that Lennie has to die at the hands of his best friend. Steinbeck knew from the beginning that George was going to need a gun for the final chapter, and he established that Carlson had a German Luger which he kept under his mattress. When Carlson shoots Candy's dog and then cleans his Luger, George is able to see exactly how this foreign automatic pistol works.
One was an accident triggered by irrational fear; the other a mercy execution. One was pure animalism; the other rationalism, a thoughtful, measured decision by a responsible adult, George.
Human beings are, first and foremost, animals, inseparable from this biological fact. Fear arises in the medulla oblongata section of the brain, often referred to as the "lizard brain." This is where animal urges are governed, like sex, fight-or-flight, survival. Hence, Lennie's commission of murder was animal in origin. There was no premeditation or considered thought, just a reflex reaction to instant terror.
George's execution of Lennie was calculated, premeditated. A plan was carried out after discussion with Candy. A different part of the brain, the cerebral cortex, was involved, which is where logic and reason are carried out.
In the courts, premeditated murder receives a greater penalty, recognizing the difference between a crime of passion and a crime of premeditation.