You have asked a really interesting question, as to me, both stories are very similar in some ways. Clearly one major way to compare them is to focus on setting. "The Destructors" is set in a post-blitz London - a wasteland of ruined, destroyed buildings, and "The Sniper" is set during the bitter civil war of Dublin in the 1920s. Both sets of characters have lived, breathed and known little else except for violence for a large part of their lives, and thus it has formed who they are. Thus we are presented with characters who, in many ways, are able to commit terrible crimes from our perspective, with great ease.
This seems to suggest that war has a tremendously negative impact on our morals and values - in both of these short stories we are presented with warped individuals, like T. and like the sniper, who, as part of their everyday lives, commit atrocious acts, such as the killing of the informer and his own brother, and in the case of T., destroying a man's house for no real reason at all except as a symbol of how he rejects the values of society. It shows us how dysfunctional war can make us and how it divides us from our own moral basis and core values. Above all, it shows us how our own bodies and minds and souls become a war ravaged wasteland. I guess one of the key differences between the two stories is that the sniper realises this and grieves, whereas T. is way beyond any sense of grief for his detached state.