The main change that George undergoes is that he finally decides that he cannot control Lennie’s destructive potential. In the beginning of the story, we are introduced to the fact that George and Lennie often get run out of wherever they are.
In chapter 1, George gets angry remembering why they are on the run this time.
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. She yells and we got to hide in a irrigation ditch all day with guys lookin’ for us, and we got to sneak out in the dark and get outa the country. All the time somethin’ like that—all the time. I wisht I could put you in a cage with about a million mice an’ let you have fun.” (ch 1)
When Curly is targeting Lennie, George encourages Lennie to defend himself but then regrets it when he realizes he cannot control Lennie.
George shouted over and over. “Leggo his hand, Lennie. Leggo. Slim, come help me while the guy got any hand left.”
Suddenly Lennie let go his hold. He crouched cowering against the wall.
“You tol’ me to, George,” he said miserably. (ch 3)
After Lennie accidentally kills Curley’s wife, George changes his mind. He decides to kill Lennie.
“No,” said George. “No, Lennie. I ain’t mad. I never been mad, an’ I ain’t now. That’s a thing I want ya to know.” (ch 6)
George does kill Lennie partly out of mercy, to prevent the law or the other ranchers from getting him. However, he also realizes that this is a mistake that neither Lennie nor George can come back from.