This is a good question, because, at first glance, this story of suspense does not seem to contain anything that could be argued to be a symbol. However, I think there is a case to think of the body of the sniper's enemy as a symbol of a much wider issue: the horrors of civil war and how it divides families and rips countries apart.
To me, one of the most significant parts of the story is when we are told how the sniper feels after he has vanquished his opponent, and he sees his enemy's corpse fall off the roof and onto the ground "with a dull thud." The next paragraph, to my mind, is one of the most important in the whole tale because it effectively foreshadows the revelation at the end:
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.
Here we see the emotional impact of the civil war on the sniper himself as he begins to realise the horror of what is going on. In his weakened, emotional state he becomes full of "remorse" and begins to curse "everybody". This leads up to the final revelation of how he has actually killed his brother, but it also, to my mind, acts as a wider symbol of what the author is trying to communicate: civil war destroys whole nations but it also operates on a much smaller level, destroying families and pitting them against each other.