The main theme of 'The Sniper' can be read as commenting on 'Division.' Another word for this in political terms is 'Partition.' We have seen in other countries besides ireland how bloody, painful and divisive political partition of a country can be. India and Pakistan endured a bloodbath after the leaving of the British.
In this short story 'The Sniper' the division is brought home to us in a truly shocking way. Many families had members who took different views, different sides. The rebels were spat at by their own people the morning they were rounded up to be shot, as many Irish could not see the sense in ending British rule. This would have been shouted about in many a home and sons, fathers, grandads, uncles and mothers would come down a particular side - and not always the same one in any given family.
The shock element drives home the disadvantages of division/partition and the sacrifice that many freedom fighrters made when the sniper turns over the body to see its identity.
A fascinating backstory of the bloodshed and struggle is provided for you here: