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I actually have to disagree with this statement. There is nothing "gradual" about the transformation that occurs in the protagonist of this excellent war-time tale. Rather, the change that he experiences is sudden and is suffered ironically as a result of his victory over his anonymous opponent. It is ironic because we would expect for such a "fanatic" who is evidently experienced enough to not be phased by killing others in cold blood to be overjoyed at his victory and his cunning over his opponent. Instead, note how the text indicates that his change is sudden and unexpected. Having watched his enemy fall from the opposite roof and land on the floor with a thud, the sniper experiences a sudden, biting remorse:
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.
Thus we can see that the undoubted change the sniper experiences is not gradual, but sudden, and seems to show that the "fanatical" face of the sniper has broken down after the release of tension following his own victory over his anonymous enemy. The discovery of the identity of his enemy seals this transformation.