The subject matter that is present in "A Horseman in the Sky" but absent in "The Death of Reynolds" is:
a young man's choice of which side to join.
In "A Horseman in the Sky," the protagonist, a young Confederate soldier, grapples with his loyalty and the impact of his choices during the Civil War. This theme is central to the narrative, whereas "The Death of Reynolds" focuses more on the consequences of war without this particular aspect of choice.