I have the feeling that "Of Mice and Men" is a fragment of a bigger novel that Steinbeck intended to write but gave up on for some reason. It seemed to promise being a story about the lives of itinerant agricultural workers. One of the things that seems lacking is detailed descriptions of the hard labor of the men and the horses in the fields. There are plenty of horses, but they do nothing but nibble hay, stamp their feet, and jingle their harnesses. The men spend most of their time indoors, either playing cards or talking. Even when they pitch horseshoes outside, we only hear the thuds and clangs.
Whenever a story ends with somebody getting shot, I always suspect that the author had run out of inspiration and just wanted to get rid of the thing so he could go on to something else. It seems significant, but it really isn't: it is just an "effect." It is melodramatic. Perhaps Steinbeck realized that he had bitten off more than he could chew--as frequently happens to creative writers. Perhaps he had to face the fact that he just didn't know enough about the subject to describe it realistically--especially to describe scenes in which gangs of men with teams of horses are working out in the flat California fields. Steinbeck does a beautiful job of describing a little campsite by a river with the mountains in the background. He does an effective job of describing a bunkhouse. Some of his characters seem real--but he could have met them in bar rooms. He didn't have much of a plot to work with. That was another one of his problems. When he wrote The Grapes of Wratha few years later, it was a full-fledged epic, and it had its own natural movement of dispossessed people moving from east to west. Of Mice and Men, by contrast, seems claustrophobic.