The sniper must find and kill the enemy sniper or he himself will be killed. His duty is not a luxury that he can decide not to perform. He is reminded of this when he lights his cigarette and is immediately fired upon. In O’Flaherty’s description of the sniper’s emotions and actions after he kills his target, O’Flaherty shows the duty that the sniper is obligated to execute.
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 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. His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.
These lines show that the sniper isn’t just a fanatic, but a soldier carrying out his duty.
I thinks he wants the reader to view the sniper as very well trained sniper who was doing is job at the right time but at the wrong place because he killed is brother.