What is the significance of the final line in Of Mice and Men, especially given its context?

"Now what the hell do you suppose is eatin' them two guys?"

The fact that Carlson has no idea what is "eating them guys" suggests a few things. Not only is Carlson callous, as mwestwood has described above, but his lack of insight also indicates that George will not be caught for the crime he has committed. 

At this point in the novel not only has Lennie killed someone, but George has, too. George's act of killing is arguably justified, but it is only justified by what he knows (and what Slim surmises). 

Slim understands that Lennie has probably killed before. When Curley's wife's body is discovered and George sees that it was Lennie who killed her, Slim connects the dots. George nods when Slim suggests that Curley's wife's death is "like that time in Weed you was tellin' about." 

Only George and Slim understand that George has killed Lennie to put an end to the danger he poses to others. Carlson believes that George has killed Lennie for the same reason that Curley would have or that he (Carlson) would have -- as retribution for the death of Curley's wife. 

Thus when Slim says "A guy got to sometimes" as a way to soothe George after killing Lennie, there is a recognition that George has killed his best and only friend and that George will have to live the guilt of this act. Despite the idea that this action was justified, George is now implicated in Curley's wife's death. Had he stopped Lennie earlier by turning him in to the authorities for what happened in Weed, Curley's wife would still be alive. 

George's guilt then is double. He has let someone be hurt by Lennie and has been forced to kill his friend as a result.

"The grief he feels over the necessity of killing Lennie is also evidence of George's essential decency" (eNotes).

What is "eating" George is highly personal and, because people like Carlson do not understand, will remain a personal guilt and a grief that he will have to find a way to live with.

What is the significance of the final line in Of Mice and Men, especially given its context?

"Now what the hell do you suppose is eatin' them two guys?"

Clearly, Carlson's callousness, shown earlier in the novella, Of Mice and Men is again evinced in his remark to Curley after Carlson is too late to shoot Lennie with his Kruger.  Just as he has had no feeling for Curley's old dog, thinking nothing of shooting the animal because "he stinks," and is no longer useful, Carlson has no pity or feelings of any kind for George, who Slim consoles.  In fact, Carlson exemplifies what George has told Slim earlier in Chapter 3:  men who

"go around on the ranches alone....after a long time...get mean."

This disparity between the reactions of George, who has lost his  friend and the dream of their owning a ranch together, and Slim, who hears more than is said and understands so much with his "God-like eyes" is sharply apparent at the end of Steinbeck's novella.  The brutal and callous like Carlson and Curley will never comprehend the fulfillment of the fraternity of men that Lennie, George, and Candy understood.  And, because of this incomprehensible callousness, men will remain alienated and solitary.

Why is the last line of Of Mice and Men fitting?

The last line of the novel is Curly's response to George as he and walks away, with Slim consoling him after Lennie dies. Curly says, “Now what the hell ya suppose is eatin’ them two guys?” This line is fitting because it contrasts Curly's lack of commitment to anyone with the devotion that George shared with Lennie. George and Lennie's friendship was stronger even than Curly's relationship with his wife. It is significant because it shows that self-absorbed people usually don't commit themselves to others or have strong relationships because they are too wrapped up in themselves. He is so inconsiderate that he can't understand why someone will mourn the loss of Lennie because Lennie didn't have anything that appealed to Curly. The line is also fitting because it was the way most of the world viewed Lennie. He was different, so most people thought he couldn't contribute anything, when in reality, he made a huge impact on George and eventually Slim.

What is the significance of the last line in Of Mice and Men?

The line at the end of the novel can be interpreted to mean that Slim and George are close friends now.

George and Lennie have always been friends, but they could never be equals.  George was Lennie’s protector.  When George met Slim, he was impressed with the fact that George and Lennie went around together.  Most men on the ranch were loners, always going from one place to another by themselves and never staying one place for long.

Slim seemed almost jealous for what George and Lennie had.  He was an impressive guy who had a good reputation on the ranch.  He interrogated George and Lennie from the beginning, curious about how they could travel in a pair and look out for each other.

"You guys travel around together?" His tone was friendly. It invited confidence without demanding it.

"Sure," said George. "We kinda look after each other." (Ch. 2)

What George and Lennie have is unique because Lennie is completely dependent on George.  George looks out for him, protects him, and defends him.  He also gets him out of trouble.  When Lennnie gets into trouble for killing Curley’s wife, George has more trouble than he can handle.  He has come to trust Slim, and Slim is the one who tries to comfort him.

Slim came directly to George and sat down beside him, sat very close to him. "Never you mind," said Slim. "A guy got to sometimes." (Ch. 6)

Slim understands the relationship between Lennie and George better than anyone else.  The other men assume that Lennie was dangerous and just tried to kill George, but Slim knows better.  He knows Lennie would never attack George, but that George had to do what he had to do to protect Lennie.  He tells George they will get a drink to show him that he is on his side.

The last line of the book indicates that Slim and George understand each other, but the other men on the ranch do not.  This puts Slim and George in a special kind of relationship, the friendship that both of them need.  They have the potential to go around together now like George and Lennie did, or George can stay put for a bit.

What is the interpretation of the final line in Of Mice and Men?

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.