Slaughterhouse-Five's mood varies sharply as the time and place changes, jumping from war time to wedding night and back again. Vonnegut's anachronistic structure frequently places clashing moods against one another. Underneath Billy Pilgrim's facade of learned nonchalance, however, there is an overarching surreal and tragic mood.
Often knowing with certainty what will happen and when it will happen due to the way he experiences time, Billy Pilgrim is frequently subdued, somewhat removed from the present. The story portrays both the very good and very bad moments in Pilgrim's life, although there often seems to be far more bad than good. Unlike the fourth-dimension-viewing Tralfamadorians, humans are often incapable of simply ignoring the bad times, however much Billy Pilgrim tries to do so. Likewise, the story forces the reader to confront the terrible evil of war, creating a dark mood somewhat lightened by the comedic tone and the determinist perspective privileged by the novel.
Overall, allusions to religion gesture toward forgiveness: for example, Kilgore Trout's rewriting of the crucifixion. The characters on both sides of the war, and from vastly separate planets, are likewise described with empathy and understanding. The humanizing perspective of the narrator then ultimately cuts through the otherwise surreal mood.