Answer:
Immediately after shooting the enemy sniper, the protagonist sniper feels extreme remorse. The text specifically says remorse. "He became bitten by remorse."
Step-by-step explanation:
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. 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.
"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."