What facts are told directly about the sniper? And what can you infer about his character?
We are told a number of things about the sniper, and from those and his actions we can infer more about his character.
We are told what his face looks like ("thin and ascetic"), and his that his eyes gleam coldly, like a fanatics. When he gets excited, he does not eat. We are told he smokes, that he's injured in the course of the story, and that he is frightened, then that the fear goes away. He's an Irish Republican.
From these facts and the action, we can infer the following:
He's completely dedicated to his cause, so much so that he'll kill without confirming the identity of those he's killing.
He's reflective about the risks he faces, but not fully wise. (He does smoke, despite the risk of exposure.)
He's a good shot.
He has a high pain threshold.
Either his family is split on Irish political issues, or either he or his brother misidentified their targets.