"The Sniper" is a short story about the Irish Civil War, fought between Irish Separatists and those loyal to the British (via the Anglo-Irish treaty). The first major theme in this story is that war is such a violent, horrifying event that it can turn a "student, thin and ascetic," into a killer. It is supposed that the sniper was a young student when he began fighting in the war. During the course of his fighting, he has become a calculating killer. Moments of his humanity surface, notably when he sees his opponent fall from the roof.
The lust of battle died in him. He became bitten by remorse. The sweat stood out in beads 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.
However, the next bullet and a drink from his whiskey flask sends him back into that reckless soldier's mentality. The sniper is trying to keep himself desensitized so that he will be able to carry out his work as a soldier. The horror of war is expressed by this first event: turning an innocent young man into a killer. The insanity of war is expressed with even greater significance when the sniper looks at his enemy's dead body and sees "his brother's face."
Because this is a civil war, the idea of fighting one's brother could be as literal as it is metaphoric. Going by the metaphor, any war is fought between brothers because we all share a common humanity.