Curley and Carlson, unlike Slim, with his "God-like eyes," and George, "small and quick,with restless eyes and sharp features" whom they watch walk away, represent the isolated men. These are the men who have spent too long alone, and the predatory features of their nature have emerged. Curley and Carlson do not understand, as Slim does, that the dreams of Lennie and George are what have sustained them and protected them from the inhospitable world of men such as they. This line underscores the theme of distrust that arises from the alienation and loneliness of the itinerant workers of the Great Depression.
It is fitting that these men stand, "looking after" George and Slim, for they represent the callous, insensitive, brutal, and violent destructivess of isolated man that George must now face without Lennie to share his "dream" in John Steinbeck's Of Mice and Men.
At the end of the book, Slim and George are walking away. They are sad because of Lennie being killed. Curley and Carlson watch them go and Carlson says "Now what the hell ya suppose is eatin' them two guys?"
To me, the meaning of this is that Carlson has no feeling for the emotions of other people. He is cold and uncaring, as we saw when he pushed Candy to kill his dog that he loved so much.
In my opinion, Carlson represents the society at large. The society does not really care about the dreams and feelings of people like George and Lennie. This is what makes life so bad during these times.