It is clear that the ending of this excellent war time story is very abrupt, and many argue that it is too abrupt. However, let us remember what the author seems to be trying to argue through his story. He is showing how first of all war transforms those involved in it by turning a student into a violent fanatic who is capable of shooting people in cold blood. Secondly he is showing that wars actually only serve to turn us against what is nearest and dearest to us. If we look at the text carefully, we can see how the ending is foreshadowed by the sniper's response to seeing his opponent fall off the roof:
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.
It is clear that at some level at least, the sniper is aware of how his actions have effected him. The beauty and genius of this story is that the actual physical realisation of this only comes at the end of the story, in the very brief and short sentence that greatly adds to the impact of the ending and the message that the author is trying to communicate.
This question has already been asked and answered here on eNotes. Here is a link for you: http://www.enotes.com/sniper/q-and-a/storys-climax-202763