Why do you think "The Sniper" is so universal and is admired?

One of the reasons why "The Sniper" is universally admired is because of its statement on war.

The main character is focused on his wartime mission.  Nothing gets in the way of his purpose.  He is described as "thin" and "ascetic" as a consequence of his war training, a training that dehumanizes sniper and enemy.  The sniper fails to find happiness:

The sniper looked at his enemy falling and he shuddered. The lust of battle...

One of the reasons why "The Sniper" is universally admired is because of its statement on war.

The main character is focused on his wartime mission.  Nothing gets in the way of his purpose.  He is described as "thin" and "ascetic" as a consequence of his war training, a training that dehumanizes sniper and enemy.  The sniper fails to find happiness:

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..... His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.

He looked at the smoking revolver in his hand, and with an oath he hurled it to the roof at his feet. 

In the middle of the story, after being wounded, the sniper experiences a short-lived moment of revelation about war and his role in it.

"The Sniper" speaks to the idea that no war is ever "civil."  When the sniper comes to see that he has killed his brother, he is left in silence.  

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

The sniper is face-to-face with the reality that he is his brother's murderer. This is the story's ultimate statement. Even though it is set amidst the Irish Civil War, it has universal applications because war makes either the sniper who kills or the brother who is killed.