In Of Mice and Men, was George right or wrong to shoot Lennie?

In John Steinbeck's novella Of Mice and Men George is forced to kill his friend Lennie after Curley's wife is discovered dead in the barn on the ranch where George and Lennie have come to work. Even though Lennie doesn't realize the gravity of his actions, George knows that his friend has committed murder and that Curley, the other men on the ranch and the law will not treat Lennie kindly. All Lennie knows is...

In John Steinbeck's novella Of Mice and Men George is forced to kill his friend Lennie after Curley's wife is discovered dead in the barn on the ranch where George and Lennie have come to work. Even though Lennie doesn't realize the gravity of his actions, George knows that his friend has committed murder and that Curley, the other men on the ranch and the law will not treat Lennie kindly. All Lennie knows is that he's "done another bad thing." George meets Lennie in the prearranged spot next to the Salinas River. While he is again describing the dream of the farm, he shoots Lennie in the back of the head. His actions are justified by the circumstances surrounding the incident.

The episode is foreshadowed earlier in the book when Candy's old dog is put out of its misery by Carlson. Candy regrets his decision to not kill the dog himself. He tells George in chapter three:

“I ought to of shot that dog myself, George. I shouldn’t ought to of let no stranger shoot my dog.”

George must have been thinking about this when Curley's wife is found with her neck broken in chapter five. Curley is enraged and makes belligerent threats toward Lennie:

Curley came suddenly to life. “I know who done it,” he cried. “That big son- of-a-bitch done it. I know he done it. Why—ever’body else was out there playin’ horseshoes.” He worked himself into a fury. “I’m gonna get him. I’m going for my shotgun. I’ll kill the big son-of-a-bitch myself. I’ll shoot ‘im in the guts. Come on, you guys.” 

Remembering Candy's words, George takes Carlson's Luger, the same gun that was used on the dog. His decision to kill Lennie is reinforced by Slim who understands the relationship between George and Lennie. Slim also knows that Lennie would never understand what was happening to him if he was captured by Curley or if he was taken to jail. Slim says,

"But Curley’s gonna want to shoot ‘im. Curley’s still mad about his hand. An’ s’pose they lock him up an’ strap him down and put him in a cage. That ain’t no good, George.”

George, then, is merciful in killing Lennie at the end of the book. Before the shot, he tells Lennie to look across the river and imagine the farm. He also reassures the big man of the importance of their friendship and George expresses his notions of how he wished things could be. He says,

“You . . . . an’ me. Ever’body gonna be nice to you. Ain’t gonna be no more trouble. Nobody gonna hurt nobody nor steal from ‘em.”

Only Slim understands George's actions toward Lennie. Slim is ever the realist and he knows Lennie needed to be put down. He tells George in the final lines of the novella:

“You hadda, George. I swear you hadda. Come on with me.” He led George into the entrance of the trail and up toward the highway.