The persona of the sniper is purposely rather vague so the audience can make their own determinations about his emotions and thoughts. All we can do is make inferences about him from his actions and an occasional description from the author. We know that he is an experienced soldier who "is used to looking at death". He is fighting for the Republican side of the Irish civil war. We are told that his "eyes have 'the cold gleam of the fanatic.' ”
In addition, he seems to be a very good soldier and even dresses his own wound in spite of being in terrible pain. He is very methodical and makes few mistakes. He lights a cigarette at night which draws attention to himself and he runs into the street where the enemy can fire upon him. But we know him only through his actions. His thoughts are not disclosed so the audience is left to fill them in with their own thoughts.
We also know that once the sniper reflects on the three people he has killed, he must feel some kind of guilt because he curses the war. When he discovers the ultimate irony in the story, that he has killed his own brother, we still do not know the sniper's thoughts. However, we do experience the horror he must have felt at making this discovery. This keeps the audience focused on the themes of the irrationality of war and the sense of isolation that war can bring.