Lennie returns to the pond after killing Curly's wife, just as George had instructed him to do. Shortly thereafter, George arrives at the site and shoots Lennie with Carlson's Luger. When the other men from the ranch arrive at the scene, only Slim understands what has just happened. George lets the men think that Lennie had taken Carlson's gun and that George had killed him when he took the gun from Lennie.
George's shooting of Lennie is foreshadowed by Carlson's shooting of Candy's dog. While Candy had allowed someone else to kill his dog, George takes responsibility for "putting Lennie down." Slim affirms George's obligation to Lennie when he says, "You hadda, George." Had George not killed Lennie when he did, mercifully and peacefully, Curly and his posse would have tormented Lennie before killing him.
The story comes full circle-they end where they started, at the lake. George faces the same choice that Candy did-with his dog. He knows that there is no saving the situation. And unlike Candy, George will not let someone else do the job. He wants to give him one last reminiscence of their dream, and shoot him quickly, which is something that he knows would not happen if those pursuing Lenny catch him.