Your original question infringed enotes regulations by asking way too many questions, so I have edited it down to one. It is a very interesting question, as it assumes that the sniper in this story is "sympathetic", which you could argue that he definitely is not. Note how he is first described:
His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of a fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.
It is incredibly important to identify the paradox the author has created - although the sniper appears but a normal student, what betrays that the war has turned him into something incredibly different - a hardened killer - is the description given of his eyes, that make him appear a "fanatic." This impression is confirmed as we see the sniper cold-bloodedly kill both a soldier and a woman without guilt, and then successfully win a battle of wits to kill his opponent, the sniper on the other roof.
However, I believe there is sufficient evidence to argue that the impression we as readers have of the sniper changes as the story progresses. A key paragraph, in my mind, comes straight after he has killed the other sniper:
The sniper looked at his enemy falling and he shuddered. The lust of battle died in him. He became bitten by remorse. The sweat stood out in beads on his forehead. Weakened by his wound and the long summer day of fasting and watching on the roof, he revolted from the sight of the shattered mass of his dead enemy. His teeth chattered, he began to gibber to himself, cursing the war, cursing himself, cursing everybody.
This shows that although he may appear a hardened killer, actually, beneath this seemingly impervious exterior the real thin, scared and guilty student lies, remembering his humanity and those of his victims. This quote also effectively foreshadows the shocking end of the story. Both of these elements could be used to argue that the sniper is a character we could - in some ways - feel sorry for.