I think the sniper believes in what he's doing, as he's described as having

. . . eyes [that] 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 seems, then, to be committed to his cause, to believe in it and his place in it so thoroughly that he is capable of carrying out even one of the most difficult things a person can do: killing other people. He is also described as having "been too excited to eat" all day, and he even decides to "take the risk" of lighting and smoking a cigarette, though he knows enemies are watching him and that the flicker of light will give away his location. This description makes it seem as though he is still pretty youthful, so much so that his nerves can still get the better of him, and so does the fact that he is willing to take a life-threatening risk for a pretty silly reason. Older, more seasoned fighters would likely be better at calming their own nerves and would not take such a risk. Thus, the sniper feels powerful and somewhat invincible.

In my opinion, he is pretty ambivalent about what he is doing.  By that, I mean that he cannot really make up his mind.  At times I think he is fine with what he is doing, at times I think he is not.

I think he is fine with what he is doing, for example, when he kills the woman who has been informing.  He would not have needed to do that and no one would have known if he had not.  So he must have done it because he really approved of what he was doing.

But then after he shoots the other sniper, he gets all freaked out and feels remorse.  That makes it clear that he doesn't always approve of what he's doing.

And I imagine that at the end of the story he's pretty unhappy with it...