How does the resolution of the conflict of "The Sniper" help reveal the theme?

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.