The sniper is a seasoned soldier, one used to seeing death on a regular basis. It doesn't phase him and his veteran status can be seen all over his face as evidenced by the following quote from the story:
"His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death."
However, after finally killing the soldier on the opposite rooftop and watching him fall to the ground, something changed inside him. He no longer had the thirst and hunger for killing that had gotten him through years of battle. For the first time in his life, he actually felt sorry for the soldier he had just killed:
"The sniper looked at his enemy 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."