In "The Sniper", why does the author end the story at this point, without showing us the sniper's reaction?

You have asked a very perceptive question that explains
the power that this short story has. The ending is certainly a definite sting in the
tail - it shocks us and makes us sit up, especially as the last sentence is given a
paragraph all to itself and is very short and
precise:

Then
the sniper turned over the dead body and looked into his brother's
face.

Of course, the way that
Liam O'Flaherty chose to end this excellent short story goes to the very heart of the
theme or meaning of the tale. This story explains and describes first hand the tragedy
and disaster civil war is, especially when it pits brother against brother and family
against family. Key to how this theme is developed is how the ending is foreshadowed in
the text. A vital part of the text for me is when the sniper is victorious and kills his
opponent. We would expect him to be happy, yet this is how he
responds:

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.

Given how he has
just shown his ingenuity in triumphing over his enemy this is a great surprise. It
indicates that at least at some level the sniper has recognised how civil war brings
death and destruction to everyone - not just the defeated but the "victors"
too.