In the story "The Sniper," what are some words and phrases the author used to describe the Republican sniper and what he's doing?

The Republican sniper, the main character in the story, is described as a young man with an innocent face, but he has the intense look of a soldier: 

His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death. 

He is young. Consider that before the war, he would have had no knowledge of the kinds of violence one sees in a war. He would have been innocent, naive, and perhaps full of hope. The author is trying to show the transition from innocence to "a man who is used to looking at death."

When the author describes him, he uses the word "ascetic" which means having extreme self-discipline. The sniper is constantly fighting for his life. He has learned this self-discipline out of necessity. It is kill or be killed and this is why he does not hesitate in killing the woman. The "cold gleam" in his eyes is the result of intense anxiety and awareness of everything around him. The author wants to make it clear that this was once a young, innocent man ("face of a student") who has now become a killing machine. This shows the dramatic and traumatic effects a war can have on the psyche. 

When the sniper is shot in the arm, he endures the pain like a professional and plans his escape. But note that when he kills his enemy and the enemy hits the ground, the sniper does break out of his single-minded soldierly mentality. "His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody." His humanism comes back to him for a moment. But the moment is fleeting. The violent sound of his revolver accidentally going off brings him back to that soldier's mentality. It is the violence that has conditioned him to become a soldier with "the cold gleam of the fanatic."