The only incident in which Lennie could be said to have made a good decision was when, after he had accidentally killed Curley's wife, he followed George's advice to hide in the brush next to the Salinas river - the area in which they slept before coming to the ranch.
He and George had spoken about him having to do this if he should get into trouble:
George motioned with his spoon again. “Look, Lennie. I want you to look around here. You can remember this place, can’t you? The ranch is about a quarter mile up that way. Just follow the river?”
“Sure,” said Lennie. “I can remember this. Di’n’t I remember about not gonna say a word?”
“’Course you did. Well, look. Lennie—if you jus’ happen to get in trouble like you always done before, I want you to come right here an’ hide in the brush.”
“Hide in the brush,” said Lennie slowly.
“Hide in the brush till I come for you. Can you remember that?”
“Sure I can, George. Hide in the brush till you come.”
Even though, tragically, George shot and killed Lennie when he found him there, it was good that he was not found by Curly or any of his men. They would have, in all probability, tortured and killed him in a most horrific way. Lennie would have suffered terribly at their hands. Candy mentioned:
“We oughta let ‘im get away. You don’t know that Curley. Curley gon’ta wanta get ‘im lynched. Curley’ll get ‘im killed.”
His death by George's hand was the most humane thing that George could do. He realised this and said:
“I ain’t gonna let ‘em hurt Lennie.
Lennie did not have the mental capacity to make decisions and he was reliant on George for his choices. George was his mentor and his guide - he had taken it upon himself to be with Lennie and they had formed a strong, brotherly bond - a relationship both men relished in his own way. It was tragic that Lennie's inability to realise his own strength and therefore control it, should have led to not only his death, but also the death of George and Candy's dream - a dream they had shared with Lennie.