I think we can point towards two main tragedies in this excellent and shocking short story. The first concerns the way in which war transforms those who participate in it, and the second concerns the way in which often war causes us to fight against the very things and people that we love.
Note the way that the sniper who is the focus of this excellent short story is presented and how it is clear that fighting in this civil war has transformed him massively:
His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.
This man should be a student and living his life as every other student does. However, the war has transformed him into a "fanatic." His actions in participating in the war have given him eyes that are "used to looking at death."
Secondly, and most obviously, the way in which the sniper finds himself ranged against an enemy who turns out to be his brother indicates the way in which civil wars and wars in general can divide cities, villages, families and even brothers from one another, setting them against each other with tragic consequences.