George did the right thing when he shot Lennie because Lennie did not understand what he had done wrong, and he would have been attacked by the other men in the worst case and arrested in the best case.
Lennie was a mentally impaired man who was very big, and did not know his strength. He travelled around with George, who was smaller but looked after Lennie. George cared about Lennie and made sure that Lennie was taken care of. He also tried to keep Lennie out of trouble, but that was next to impossible. The best George could do was try to contain the damage, and protect Lennie from the consequences as best he could.
When Lennie was run off for trying to touch a lady with a dress, it foreshadowed the trouble he got into with Curley’s wife and it was a warning to George.
"Jus' wanted to feel that girl's dress- …- Well, how the hell did she know you jus' wanted to feel her dress? She jerks back and you hold on like it was a mouse. … All the time somethin' like that- all the time. (Ch. 1)
George is able to get them work because Lennie is strong. As long as they get the job before Lennie opens his mouth, and keep it once they see Lennie work, they will be okay. But George knows as soon as he sees Curley’s wife that there will be trouble. He thinks the trouble will come from Curley, who he sees as wanting to prove he is tough. He warns Lennie to stay away from Curley.
"Look, Lennie! This here ain't no setup. I'm scared. You gonna have trouble with that Curley guy. I seen that kind before. He was kinda feelin' you out. He figures he's got you scared and he's gonna take a sock at you the first chance he gets." (Ch. 2)
The trouble the George fears does materialize, and Lennie almost crushes Curley. Curley picks a fight with Lennie because he says that Lennie was laughing at him, when Lennie was doing nothing of the sort. Lennie grabs his hand out of fear and does not let go. The story they decide to go with is that it was crushed in a work accident, but George knows that trouble is brewing. He has no idea.
The real trouble comes with Lennie is sitting in the barn and Curley’s lonely wife comes in to see him. He pets her pretty hair and accidentally breaks her neck. George realizes that Lennie is not just big and harmless. He actually is dangerous. He knows that the men will come for him, and that Lennie does not understand what he has done. He did not mean to kill the girl. The men will attack him, and hurt him. Lennie will be scared, and sad, and George will be helpless to do anything to save him.
"Yeah," said George. "I'll come. But listen, Curley. The poor bastard's nuts. Don't shoot 'im. He di'n't know what he was doin'." (Ch. 5)
George is also aware that if he can prevent the men from seeking vengeance on Lennie, the law will take over. Lennie will go to prison, where he will be scared and alone and also not understand what is happening to him. George does not want to do it, and it breaks his heart, but he knows what he has to do. It is his duty, to his friend. He shoots him so no one else will
George decides to put him out of his misery, like the men do with the dog. They put him down, which is the humane choice. George puts Lennie down, like a master to a beloved dog who has bitten someone.
George must have thought he was doing the right thing or he wouldn't have done it. All the men in the lynch mob must have thought they intended to do the right thing in planning to kill Lennie themselves. George was actually part of the lynch mob, only he got to Lennie first because he knew where Lennie would be hiding. He knew because he had told him where to hide if he got into trouble. What the men in the lynch mob all thought was that Lennie had tried to rape Curley's wife in the barn and had killed her, either accidentally or deliberately, to keep her from screaming for help. That was actually not too far from the truth. Lennie was sexually attracted to the girl. He started off by stroking her hair, but then he was holding her by a fistful of hair, which certainly suggests that he was becoming aroused. Lennie didn't understand his own feelings with Curley's wife any more than he did when he was, not just feeling, but grabbing and pulling at that girl's dress in Weed. In both cases the assault could have led to rape or attempted rape. Lennie was stopped in Weed by George hitting him over the head with a fence picket. Lennie was stopped in the barn by his realization that Curley's wife was dead.
No one else was present in the barn. Only the reader knows what happened. I think most readers would agree that if Curley's wife had not screamed and struggled, and if there had not been a number of men pitching horseshoes right outside the barn, Lennie would have tried to tear the girl's clothes off and rape her. If he were arrested he wouldn't just go to prison; he would be executed for murder. He killed the girl. He would be totally incapable of explaining what happened. He is just a hobo with no money to pay a defense attorney. George could testify that he was mentally incompetent, but that probably wouldn't help much, and it might get George into trouble. The District Attorney might even bring up the Weed incident! It should be very little trouble to trace Lennie and George back to Weed. They probably got their job up there through the same San Francisco agency. George couldn't get Lennie committed to a state mental institution because George is not a relative and Lennie is not insane.
The real killer is John Steinbeck. He wanted to write a story about the hard lives of itinerant farm workers in California. But he planned to turn the story immediately into a stage play. Both the book and the play came out in 1937. That is why he wrote the book as what he called "a playable novel"; that is, a novel that reads like a play, with almost everything in description or dialogue and virtually no prose exposition or explanation. But a stage play only lasts for a couple of hours, whereas a novel about men working in the fields with teams of horses and traveling from place to place looking for work really should be hundreds of pages long. Steinbeck decided on a "shotgun ending." If one main character kills the other main character, then the story can end abruptly. This also explains why the book seems so flimsy. It can't be called a novel. It is usually called a novella. Steinbeck gave the subject the full treatment it deserves when he wrote his masterpiece The Grapes of Wrath.
Did George do the right thing? Well, he probably did the kindest thing. He proved himself a true friend, not unlike those soldiers in Shakespeare's Julius Caesar who helped Cassius and Brutus to end their lives at Philippi.
George believed that shooting Lennie was the right thing. Lennie did not understand what he had done and his intentions were not bad, but because what Lennie had done was so serious he would not have been forgiven. George shot Lennie to keep him from what would have been a much worse situation. Lennie would have been beaten and hated by everyone at the ranch without understanding why. That would have caused Lennie so much more pain than the death George gave him.