By "negative aspect," I assume you are refering to the cautionary message that this powerful text contains about the dangers of war and in particular how civil war divides street against street, friend against friend, and, most poignantly, brother against brother. This is of course communicated through the final shocking discovery of the identity of the sniper's anonymous opponent. However, what is interesting stylistically about this story is that it ends rather suddenly and abruptly. Partly this is to emphasise the shocking nature of the discovery, but partly it is because the emotional reaction of the sniper has already been given to us. In an excellent example of foreshadowing, the sniper, after seeing his victim fall to the ground dead, suffers an emotional attack which is made all the more powerful for the way that it foreshadows the identity of his opponent and also because of our expectation of his happiness at having vanquished his opponent:
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 teh 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.
The negative aspect of this story is thus shown through the emotional reaction of the sniper to killing his enemy. The profound impact of this "victory" suggests that in civil war we harm ourselves as much as anybody else.