The main theme of "The Sniper" is the tragedy of warfare. This story is about the Irish Civil War, so it makes a more specific comment on war by showing how soldiers can become desensitized to violence and, in turn, how nations can except war as a rational solution. At the end of the story, the protagonist approaches the man he's just killed, turns him over and "looked into his brother's face." Civil wars have often been described as "brother against brother." This could imply that actual brothers can fight on opposing sides of the war, but it also could imply that citizens of the same country are symbolically brothers, stating that any civil war is violence against one's own family. Taking this a step further, any war at all is violence against one's own species. Taken quite literally, any war is violence against your own kind. When the protagonist sees "his brother's face," this might be a moment when he once again becomes sensitized to death.
"The Sniper" by Liam O’Flaherty is a story written in the third person, closely following the viewpoint of a sniper on the Irish Republican side in the Irish Civil War. The narrator, although using the third person, is limited to the sniper's viewpoint and has full access to his thoughts and emotions.
The theme of the story is only fully revealed with the surprise ending. Through the story we get an increasing sense of the similarities between the two snipers, the viewpoint character and his enemy. At first, we think that the similarity and even empathy has to do with their identical roles leading to similar ways of thinking and building a bond between the two men. The sniper, after killing his enemy:
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.
This suggests that the central theme of the story is the cruelty and senseless violence of civil war. When the sniper shoots from a distance, he can separate himself from this reality, but when he draws physically close to the anonymous "enemy" he can no longer shield himself from its reality. The final revelation of the story makes us realize that people with whom we disagree should not be characterized simply as anonymous "enemies".