John Steinbeck's novel Of Mice and Men ends in a tragedy: after Lennie accidentally kills the farm's boss's (Curley's) wife, he runs to a place of safety where he knows that he will encounter George for help. This is a plan that George and Lennie have had in place since the beginning of their friendship, since Lennie is intellectually limited, and it is George who has always come to his rescue.
This time, however, Lennie is in a really bad situation because Curley has formed a mob to find Lennie and have him lynched. As a form of mercy killing, George has to make the terrible choice of shooting Lennie on the back of his head. This is done without Lennie noticing, and in order to avoid a much more cruel and painful death at the hands of Lennie.
Now, let's remember that Of Mice and Men is an allegory of a society that exists in the America of the Great Depression. The characters not only portray themselves independently, but they carry with them the weight of the many prototypes of the time. This fact has to be acknowledged in order to truly appreciate the ending of the novel, and the question that is asked when the novel ends.
This being said, let's go back to the final moment of Lennie and George's friendship, which ends with Lennie's death at the hands of George. Although the magnitude of this horrible choice is felt altogether among the rest of the farm hands who witnessed it, even Curley himself, it is Slim who comforts George in the only way that he can: by reassuring him that sometimes even a bad choice is better than a choice not made at all. Then, the story ends
Slim said, “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. Curley and Carlson looked after them. And Carlson said, “Now what the hell ya suppose is eatin’ them two guys?”
Here we see the true colors of the society which is represented in Of Mice and Men. Slim, the most promising and enlightening man among all the field hands represents the hope for a better America, or a better life; he is comfort, intelligence, and the common sense that George's unfair and chaotic world lacks.
George, who represents the American Dream now lost, has no choice but to accept his fate and, basically, declare his dream null. He represents the typical man of the Depression: one who is bound to accept his fate in every way.
Contrastingly, Curley and Carlson are the antagonists; the men of an upcoming generation of superficial, shallow, and dead values. Contrary to the other field hands, Curley and Carlson lack manliness, integrity, and a general appreciation of life. They merely "are". Therefore, the cruel and out-of-line question asked by Carlson denotes his lack of care for any other human life but his own.
Concisely, when Carlson asks the question of what could be "eating up" George and Slim, he is stating his disregard for loyalty, friendship, or the human connection of one person to another. He represents the upcoming factory-led world of bulk and heartless dynamics; an America who may turn into Carlson: shallow, empty, null. Therefore, with this question, Carlson represents the America that Steinbeck wants to warn us into never becoming.