The answer to this question is not difficult, but it is awkward. It's awkward because "The Sniper" doesn't use character names. There is a sniper, another sniper, an old woman, and a soldier in an armored vehicle.
Among those characters, the protagonist is the sniper that is fighting for the Republicans. The narration of the story focuses on this sniper, and the reader is driven to desire his survival over the other sniper. That is the main reason that the Republican sniper is the protagonist.
The other sniper is the main antagonist. He fights for the Free Staters. He is the main threat to the Republican sniper, and the Free Stater sniper succeeds in wounding the protagonist. At the end of the story, the protagonist has defeated the enemy sniper, but it is a hollow victory. The dead enemy sniper turns out to be the brother of the protagonist.
Then the sniper turned over the dead body and looked into his brother’s face.