What is the central meaning or theme of "The Sniper"?

The central meaning of Liam O'Flaherty's short story "The Sniper" is the vicious unpredictable nature of war and the cruel toll it takes on the soldiers who fight. The Republican sniper is responsible for the deaths of three people in the short time span of the story. He may have killed many more in his role as a sharp shooter on a Dublin rooftop during the Irish Civil War. 

The war in the streets of...

The central meaning of Liam O'Flaherty's short story "The Sniper" is the vicious unpredictable nature of war and the cruel toll it takes on the soldiers who fight. The Republican sniper is responsible for the deaths of three people in the short time span of the story. He may have killed many more in his role as a sharp shooter on a Dublin rooftop during the Irish Civil War. 

The war in the streets of Dublin is an unpredictable chess match. Even the lighting of a cigarette could lead to death. Soldiers and civilians alike are in danger in the chaos of battle. The sniper kills a woman who is pointing out his position, an armored car commander who foolishly reveals himself and, after staging a diversion, the Free-State sniper. The death toll, however, wears on the sniper. 

After tricking the enemy sniper into showing his position and killing him, the Republican sniper breaks down. He temporarily loses his head and almost kills himself when he throws down his pistol. The stress of battle sends him into a temporary madness. O'Flaherty writes,

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.

O'Flaherty never reveals the sniper's reaction when he discovers the dead man is really his brother. Obviously it would have been something extremely difficult to live with. The cruelty and unpredictability of the war must have been a crushing blow to his spirit.