We see the emotions of guilt and remorse the sniper feels when he kills the old woman and the soldier in the street below his rooftop perch and again when the kills the sniper on the rooftop across the street (whom, at this point, he does not know is his brother). His reaction is to throw down his revolver as though he will never touch it again, which only causes the gun to fire and nearly results in him shooting himself in the head.
However, I don't think the sniper would go one to kill himself once he learns it is his brother dead in the street. Certainly, the rest of his life will be filled with guilt, remorse, and many other emotions. I think the more likely reaction is symbolized in the throwing down of the revolver. The sniper will walk away from the war, walk away from the scene, and re-invent himself in new life forever trying to understand and atone for his actions.