What is an interpretation of the final line of the novel Of Mice and Men by John Steinbeck?

It is significant that the crass and callous Carlson says "Now what the hell ya suppose is eatin' them two guys?" For, this statement points to the naturalistic nature in man that emerges with the degradation men experience as a result of their alienation and disenfranchisement.
Just as Carlson has been insensitive to Candy in shooting his old dog because "he stinks," he also has no pity or sympathy for George, whom Slim consoles. Indeed, Carlson exemplifies the men George describes to Slim earlier in Chapter 3:

"[men who] go around on the ranches alone....after a long time...get mean."

In addition to his callousness toward other men, with this remark Carlson exhibits his crassness, as well. This lack of discernment on the part of Carlson indicates that George will escape from being charged with the crime he has actually committed. For, Carlson simply believes that Lennie was in possession of his gun and George took it away and shot Lennie in either self-defense or because he killed Curley's wife. Whichever of these it may be, the men would feel the shooting justified, so Carlson wonders "what is eating" George and Slim that they should be upset.
That which "is eating" George is his conscience because he knows that he should have done something about Lennie after the incident in Weed. Consequently, he feels guilty about Curley's wife's death as well as his shooting of Lennie.