As in many novels, the protagonist of Slaughterhouse Five, Billy Pilgrim, can be understood as a morally neutral "stand-in", taking the reader through the story without making judgments or comments on the story's events. His purpose, in a way, is to give the reader someone to identify with in the context of the story.
We can see Billy's moral neutrality especially in the fact that he doesn't carry a gun during the war (until the very end when someone makes him, and even then he almost certainly would never use it). Billy is in the war but he is not exactly a part of it.
Through him, the reader witnesses the atrocities of war, the compromises of married life and fatherhood, and the generosity of humans even in times of great distress.
Also, in this novel, Billy has a rather peculiar relationship to the narrator because the narrator is also a character in the story. This leads the reader to conclude that 1) Billy is a fiction character (or even just a fictional device) and 2) Billy's story is not his own story, per se, but is better understood as the story of the writer, of a small section of WWII and of the bombing of Dresden.