Despite the fact that the protagonist "is used to looking at death," when he sees the dead body of his enemy fall from a rooftop and hit the pavement below, he shudders and feels the "lust of battle [die] in him." He experiences a sudden remorse, an emotion we are given to believe is not familiar to him, as he is a "fanatic" for his cause who has just shot and killed two other people without any negative feeling whatsoever: a driver of an armored car and an old woman acting as an informant.
Something about the "shattered mass of his dead enemy" is now revolting to him. His teeth begin to chatter, and he begins to "gibber to himself, cursing the war, cursing himself, cursing everybody." Perhaps he does so because he recognizes, maybe for the first time, the careless brutality of war, as it turns young men into murderers of other young men. Nothing matters except for which side one fights on. So, he curses the war that does this to people—that dehumanizes other human beings—himself for allowing it to be done to him, and everyone else who participates in this reckless and suddenly baseless-seeming fight. He doesn't think of his cause now; he thinks only of his remorse and disgust.