In the story "The Sniper", what motivation induced the protagonist to investigate the identity of the other sniper?

After killing the other sniper, the sniper takes a drink of whiskey and decides to descend from the roof and try to rejoin his company. Retrieving his revolver, the sniper crawls down into the house. Once at the street level, the sniper has an urge to see the man he killed.... a sense of curiosity. He might know the man from the army before the civil war began. The sniper runs into the street, drawing a spate of machine gun fire from a distance. He throws himself on the ground besides the corpse of the enemy sniper. He turns the body over. He looks into the face of his brother.