Immediately after shooting the enemy sniper, the protagonist sniper feels extreme remorse. The text specifically says remorse.
He became bitten by remorse.
Remorse is a great word, because it encompasses a few other emotions as well. Within the definition of remorse is regret and guilt. The sniper feels badly about killing his enemy. He regrets having to have killed another skilled shooter, and feels guilty over the carnage that he has committed.
. . . 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.
After the sniper pulls himself back together, he decides that he wants to investigate who the other sniper is. It's not morbid curiosity to see a dead body up close. It's more akin to paying his respects to the other sniper. Plus, the sniper believes that there is a chance that he might know the enemy sniper.
He decided that he was a good shot, whoever he was. He wondered did he know him. Perhaps he had been in his own company before the split in the army.
It turns out that he does know the man that he killed. It's his own brother. The above details about the sniper's regret and desire to pay respects to the enemy combatant reveal that the sniper is not a blood thirsty killing machine. He is doing his job for the war, but doesn't take pleasure in it. I would say that O'Flaherty presents the sniper as a centered man and very human.