By focusing on images of destruction, brutality and the psychological derailment of the protagonist, Billy Pilgrim, Vonnegut presents a clear anti-war stance. His approach to conveying this message is not through direct condemnation because that would be ineffective and limited in scope, a sentiment echoed in the narrator's apology to his publisher about the fractured nature of the narrative: "It is so short and jumbled and jangled, Sam, because there is nothing intelligent to say about a massacre" (Chapter 1). Instead, Vonnegut sheds light on the inhumane consequences of war primarily through Billy's descriptions of the wasteland created by the firebombing of Dresden and through Billy's own psychological splintering; one possible consequence of Billy's post-traumatic stress disorder is his conviction that he was abducted by aliens and is capable of time travel. When Billy is sent to the camp hospital for uncontrollable, hysterical laughter during a performance of Cinderella, he time travels and wakes up in a veterans' mental ward after the war, where someone remarks, "My God—what have they done to you, lad? This isn't a man. It's a broken kite"(Chapter 5). This statement encapsulates Vonnegut's anti-war message: the atrocities of war and the compromises it requires of its actors reduce people to something barely recognizable as human. It is precisely this dehumanizing effect—apparent throughout the novel in the thoughtless, insensitive, cruel way people treat each other during wartime—that presents a true condemnation of war.

Slaughterhouse-Five is an anti-war novel. Vonnegut's protagonist suffers from post-traumatic stress disorder. Billy Pilgrim's psyche has become so fractured that he "has become unstuck in time." Due to the trauma he has suffered in the war, Billy is unable to live in the present.

Even before he begins to suffer from PTSD, Vonnegut is careful to portray Billy Pilgrim and all of the other soldiers in non-heroic terms. Billy looks more like a "filthy flamingo" than a soldier, and the men who capture Billy are "droolers as toothless as carp."

At one point, Billy, while watching a World War II documentary, imagines that he is seeing the film backward. Rather than dropping bombs, the bombers "flew backwards over a German city that was in flames. The bombers opened their bomb bay doors, exerted a miraculous magnetism which shrunk the fires, gathered them into cylindrical steel containers , and lifted the containers into the bellies of the planes." The bombs are then dismantled and buried "so they would never hurt anybody ever again."

Vonnegut ends his novel with the word "Poo-tee-weet?" because "there is nothing intelligent to say about a massacre." In effect, Vonnegut is saying that war makes no sense, and there is nothing intelligent to be said about it.