The ending of O'Flaherty's short story certainly presents a message which focuses on the cruelty and familial divisiveness of civil wars. When the sniper discovers that he has killed his own brother, the author clearly shows how Ireland's war with itself pitted brother against brother (much like the American Civil War). Most critics consider this to be the primary message of the story.
However, the beginning and middle parts of the story also allow the author to illustrate the mind and nature of a warrior. His almost mechanical description of the sniper demonstrate the effects of war and combat on an individual.
The central message of "The Sniper" lies in the agonizing middle of the story and is illustrated by the blood chilling conclusion of the story. The message is that war dehumanizes people. It dehumanizes people to such an extent that they completely overlook their unity of human fellowship.
We know from the introduction that at the time of the action the Republican sniper is tired and hungry, having spent all day in the hot sun on the rooftop--hidden--watching. He knows smoking a cigarette is a safety risk: in the dark, the flash of the match will be seen. Exhausted, he chooses to take the risk. Immediately a bullet "flattened itself against the parapet." The sniper's life takes an unquiet turn because of it.
He is wounded by the opposing sniper's--the Free Staters'--next bullet. To save himself, because morning "must not find him wounded on the roof," he tricks the other sniper into shooting again but this time at a false target. He is successful. The opposing sniper exposes himself and is killed by the Republican sniper.
Seeing him "crumple up," fall "over and over in space" then hit the ground takes the fight out of the sniper. He loses the "lust of battle": "The lust of battle died in him." He is "bitten by remorse." He sweats. He's weakened. He'd been fasting. He'd been scorched in the sun on the rooftop all the "long summer day." He "revolted from the sight" of his enemy shattered: "he revolted from the sight of the shattered mass...." His teeth chattered, he gibbered, he cursed. He abandoned his post to return to his unit, to his moral support, to his war lust support. As he ran, he detoured to see the dead enemy: he had a "sudden curiosity as to the identity" the man he had killed.
The message of the story meets the Republican sniper when he confirms in his brother's face what he has already begun to feel in his chattering teeth, that he has surrendered his humanity to murdering war: "The woman's corpse lay still in the gutter."
The sniper looked at his enemy falling and he shuddered. The lust of battle died in him. He became bitten by remorse. The sweat stood out on his forehead. 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 teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.