Why do you think the author chose to "call" the main character as he did?
If by "call" you mean why did the author portray the character in the way he did, with the ending the story has, my impression would be that O'Flaherty is trying to impress upon the reader, not only what war can do, but to draw attention to the terrible fighting that went on in Ireland for many, many years.
This story is about civil war in Ireland, between the Republicans and the Free Staters. As with the Civil War in this country, the politics divided families onto opposite sides of the battle.
The description the author presents of our protagonist is that of a young man, who is good at what he does, but who is also something of a fanatic. As the story begins, he is wound up and very tense. He is eating now because he has not eaten all day: he's been too nervous.
In the story, the sniper is there to do a job. He does not personify the people he kills: he is killing the enemy, and they are faceless victims. When he kills the men in the armored tank, he goes ahead and kills the old woman who tried to tell the men in the tank where the sniper was located. The fact that she is an old woman means nothing to him. The only way he can effectively do his job is to separate himself from any emotions while he carries out his duties.
When the sniper kills the old woman, a sniper on another roof wounds the protagonist. Though it does not kill him, he knows he cannot shoot his rifle again.
Rather than packing up and going to find medical attention, the sniper dresses his own wound and then methodically goes about trying to draw out the other sniper in order to kill him.
Ultimately, by tricking the other man, the first sniper is able to kill his adversary. At first, viewing this man as a fine shot and a worthy adversary, the sniper is full of rage about the realities of war and the waste of human life, and he nearly kills himself when he throws the revolver he used violently onto the ground, where it misfires.
Finally pulling himself together, the sniper climbs down from the roof and decides to see if he knows the other sniper, perhaps from an old regiment. However, when he turns the man over, he is looking into the face of his dead brother.
The ending never fails to shock and appall the reader. O'Flaherty artfully lures us into the story, keeps the excitement high, and then pulls the rug out from under us. However, he is once again trying to bring to the reader's attention that although the sniper endeavors to keep it "non-personal," this is really impossible: war is personal. To kill means to take the life of another human being. In this case, it is more horrible than the sniper could imagine, as he has taken the life of his brother.