I would want to argue that your question, as excellent as it is, needs to add one word to clearly focus on the issue presented to us in this wonderful story. Let us not forget that the war in which this story is set was a civil war, which was infamous for dividing Ireland and still lingers on today in the form of deep-seated divisions and long-standing rancour. In addition, this civil war, like so many, was characterised by its ability to divide streets and even families. This story explores this concept through the shocking realisation of the sniper at the end of the story as he turns over his defeated enemy and realises he has only killed his brother.
Of course, this realisation is symbolised by the feelings of the sniper after his victory over his unknown enemy. Note how the text describes what the sniper endures:
The nsiper 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.
Civil war is shown through the devasating ending to be something that ultimately only hurts yourself. Engaging in acts of violence against an unknown enemy is shown to be really engaging in acts of violence against yourself. Even though the sniper is shown to be "victorious," really, his victory is hollow and meaningless as he realises the cost of it and the high price that civil war always exacts from those who participate in it.