Two obvious themes are that of (1) brother pitted against brother in war and (2) the objectification and dehumanization of faceless, identity-less participants (warriors, but also victims) in war. A third more subtle yet more dramatically central theme is that the heat of war blinds participants to the reality of murdering living individuals.
To explain the first theme, the theme of brother against brother is a literal theme and a metaphorical one. In civil wars, literal brothers may choose opposite sides and face each other across firing lines. In all wars, metaphorical brothers, who are common members of the human race, face each other across firing lines.
To explain the second theme, the theme of objectification and dehumanization is metaphorical; the enemy is seen to be like nothing. The enemy is seen to have no object in their existence, no humanity in their individuality as they are hunted and shot down, whether enemy or informant, whether old or young, whether male or female, whether student or worker. This objectification and dehumanization hits both warrior and victim. The warrior abandons an innate sense of humanity in order to live "enveloped in darkness" with the "cold gleam of the fanatic" as [in this story] a sniper. The victim is seen as the "enemy" and nothing more.
To explain this third subtle and more central theme, the theme of blindness to the reality of doing murder in war is representational of the dependence of acts of war upon first obliterating all trace of fear--even fear of losing one's own life--upon obliterating the fear of living with and "looking at death."
They were deep and thoughtful, the eyes of a man who is used to looking at death.
This fearlessness, even of one's own death, is needed for the act of cold-blooded killing in war, for cold bloodedly aiming in the dark--a trained, skilled, sure-shot sniper--at an old woman, at a man, at one's own brother. The sniper represents this great fearlessness. Snipers must be, above all other modern warriors, cool, steady of nerve, impersonal, with eyes that are "used to looking at death."
This theme is dramatized when in the story the sniper has a moment of thunderous fear. Weakened by fasting and having been wounded, he recoils, "revolts" from the horror of watching his enemy die, falling, tumbling "over and over in space," from the rooftop opposite.
Weakened by his wound and the long summer day of fasting and watching on the roof, he revolted from the sight of the shattered mass of his dead enemy.
His recoiling, revolting horror and his fear--of what he has done as much as of what might be done to him--compel him to "hurl" his revolver down at his feet. It goes off, sending a bullet past him.
The shock of the gun's report and the whizzing bullet snaps him out of deep fear, dispelling from him the "cloud of fear" that exploded in his mind at the sight of his tumbling, dead enemy. His nerves steady and he can resume being "the sniper"; he can escape the fear of being a man.
The subtle though overarching theme, then, in this story is that the heat of war blinds warriors to the reality of doing murder. When the blinders are removed, as by watching a skilled man, though an enemy, tumble in his death agony, or as by looking in a dead enemy's face and finding a brother, fear is unleashed in a mushroom cloud of horror that recoils, revolts against and rejects the machines of war.
The lust of battle died in him. He became bitten by remorse. The sweat stood out in beads on his forehead. ... [He] revolted from the sight of the shattered mass of his dead enemy. ... Then the sniper turned over the dead body and looked into his brother's face.