It's the end of the movie and The Drifter has to be moving on. There are other people in trouble, other wrongs to right, other paths to follow. He saddles up his horse and rides west into the setting sun. The townspeople look on as his figure, silhouetted against the orange disk, disappears into the horizon. The music swells and "The End" appears. Riding into the sunset is a tried (some would say "tired") and true ending to a show. Primarily a Western trope associated with cowboys, but not exclusively a western trope. The setting sun is symbolic of the end of the story.