I think it's a stretch to assume the Coen Brothers intended your definition of "shot in the back" over a literal definition. I think their intention was to show that Buster is not just a hypocrite, but a man who uses the Code of the West to justify his misanthropy and antisociality. That's one way of looking at a person like this, a man of Western Virtue. On the other hand, you've got Mr. Arthur in "The Gal Who Got Rattled". He too is a remorseless and effective killer, a man who can survive on his own in the wilderness, generally a paragon of Western virtue. Unlike Buster he's ugly and dirty and hardly says a word - a more authentic trail-weary cowboy. His virtues allow him to heroically rescue Alice Longabaugh from the Native war party, but she as a city slicker loses her nerve and shoots herself in the head. Then you've got the robber and the bank teller in Near Algodones - how do those two fit into the same codes of Western virtue? Killing, self-sufficiency, courage, verbosity, and the luck of the draw - these are the big themes of Buster Scruggs.