Carlson is an insensitive ranch hand who Steinbeck uses in opposition to George. While George shoots Lennie out of sympathy and fear of what others might do to Lennie, Carlson disposes of life without regard to the consequences or sorrow that result. For example, Carlson encourages Candy, an old ranch hand who has lost one of his arms, to kill his dog. Carlson says,"Whyn't you get Candy to shoot his old dog and give him one of the pups to raise up? I can smell that dog a mile away. Got no teeth, damn near blind, can't eat. Candy feeds him milk. He can't chew nothing else" (Chapter 3). Carlson argues that the dog should be shot because the old animal is suffering, but his motives are selfish, as the dog's smell offends him.
When George shoots Lennie at the end of the novel, he does so to protect his friend, and George clearly suffers with his conscience as a result. This act is very different than Carlson's selfish suggestion that Candy have his beloved pet killed. After the dog is shot, Carlson meticulously cleans his gun, and therefore George and Lennie both know about the gun and where it is kept.
George later kills Lennie with Carlson's gun. Steinbeck writes at the end of the book about George, "He reached in his side pocket and brought out Carlson’s Luger; he snapped off the safety, and the hand and gun lay on the ground behind Lennie’s back." George's murder of Lennie is symbolically connected to the murder of the dog, as George uses the same gun to carry out his act, but George is motivated by love, not by selfishness, as Carlson was in suggesting that Candy's dog be shot.