There are two types of conflict portrayed in the short story 'The Sniper' by Liam O'Flaherty. The first is the conflict on the roof between the two fighters. The main character in the story ends that one by shooting his opponent - the irony is that that reveals another conflict in the fact that the opponent is his own brother. That in turn resolves another conflict as it means that,unless there was a mistake, the brother must have been fighting for the other side.
The second big conflict was of the course The Easter Rising or rebellion against the English occupation by Ireland in which the brothers fought. This has never been resolved! Even during the Peace Process now, arguments and debate still rage over the level of British involvement in the North. Some degree of messy resolution was achieved by partition, but only some of the southern counties achieved independence.Some people see that as a cave-in and a terrible mistake.
The Sniper isn't a traditional plot line in that there is a conflict and a resolution. It is a surprise ending that heightens the message in the story--that war is senseless because if all the death and destruction. It is horrific for a feeling man to have to kill and then, at the worst moment, when he sees he has killed his brother, it makes the tragedy of war even worse.
If I were to pin down the most critical moment in the story, I would say it is when he turns the man over and sees it is his brother.