Carlson was not to be put off. "Look, Candy. This ol' dog jus' suffers hisself all the time. If you was to take him out and shoot him right in the back of the head-" he leaned over and pointed, "-right there, why he'd never know what hit him."
Candy looked about unhappily. "No," he said softly. "No, I could not do that. I had 'im too long. I'm so used to him," he said softly. "I had him from a pup."
[Carlson] continued to look down at the old dog. Candy watched him uneasily. At last Carlson said, "If you want me to, I'll put the old devil out of his misery right now and get it over with. Ain't nothing left for him. Can't eat, can't see, can't even walk without hurtin'."...
Candy said, "Maybe tomorra. Le's wait till tomorra."
"I don't see no reason for it," said Carlson. He went to his bunk, pulled his bag from underneath it and took out a Luger pistol. "Le's get it over with," he said. "We can't sleep with him stinkin' around in here."
He put the pistol in his hip pocket. Candy looked a long time at Slim to try to find some reversal. And Slim gave him none. At last Candy said softly and hopelessly, "Awright- take 'im."
Which theme is developed in the above excerpt?