Excellent question. Clearly any discussion of the "main theme" of a work of literature is going to be debatable precisely because any work of literature can have a multiplicity of meanings and themes. However, the setting of the conflict that the sniper faces during the civil war of Ireland, which was notorious for setting community against community, street against street and even families against each other, suggests that the horrors of civil war are being commented upon. This is clearly supported by the shocking revelation at the end of the tale that the unnamed and unidentified adversary of the sniper that he eventually triumphs against is actually his brother:
The sniper turned over the dead body and looked into his brother's face.
Note the abrupt and concise manner in which the author ends the tale. This clearly adds emphasis to the ending, which, combined with the element of foreshadowing that came before, indicates the theme of the tale: civil war is a horrendous thing that breaks up and destroys communities and even families, pitting us one against the other and forcing us to commit unthinkable crimes.