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.