For several years I have associated Slaughterhouse-Five, which I first read in high school, with Thomas Pynchon's The Crying of Lot 49 (1966), which I read on a lark in graduate school. In addition to the novels' non sequitur quality, their heroes are powerless, fumbling around at the mercy of something large, strong, and incomprehensible. One key difference between Oedipa Mass and Billy Pilgrim is that Oedipa seeks answers, a place for herself in relation to an alleged worldwide conspiracy. Billy Pilgrim is so shattered by World War II and the bombing of Dresden that he retreats (not entirely consciously) into private fantasy.
In Chapter 1 Vonnegut admits that his book is a mess. This is true. As best I can tell (and this is pure, uninformed opinion), the science fiction element is imagined by the protagonist. Billy's war experience has driven him mad, giving him delusions of time-travel and visits to the planet Trafalmadore. The contours of the fantasy are determined by one particular postwar experience: the reading of science fiction in 1948, while making a recuperative stay at a mental institution. "So they were trying to re-invent themselves and their universe. Science fiction was a big help" (Chapter 5). Various pieces of Billy's life are folded into the fantasy he constructs: his father's pitiless swimming lessons, his sexual experiences, the crucifix hanging over his childhood bed. (Vonnegut's narrator has a mild, piteous admiration for Jesus; he refers to "all the lovely and puzzling things he said in the [canonical] Gospels.")
The novel doesn't have much in its favor. The fact that Vonnegut intentionally deranged his plot does not excuse it. (Unlike Station Eleven, I had no trouble stitching together the story). Much of the book is pornographic; my high school teachers ought not to have assigned it. Billy is an unforgettable everyman-soldier (his last name is "Pilgrim"; his hometown is "Ilium"), but there are no other characters in the book. Vonnegut's American women are cruel, ugly-and-stupid, or beautiful-and-stupid. His European civilians, male and female, are sophisticated, decent, and more or less innocent. This is not a great defect, though. In social satire, the characters don't matter. It is situations, incidents, and institutions that are the real object of attention.
Vonnegut's narrative voice is relaxed but never out of his control, as is clear from the incorporation into the plot of several distinctive, imaginary manuscripts: quotations from a science-fiction novelist, Kilgore Trout, and a book written by an intelligent American Nazi, Howard W. Campbell, Jr. The imitations of a variety of Englishes are good: British, immigrant American, Midwestern American.
The book has a strong and pretty good moral purpose, most evident in its responses to death. The effectiveness lies in brevity. Vonnegut creates many terrible images of mangled and suffering bodies, and refuses to linger on them. They come and go quickly, not unlike the incidents in the time-hopping plot. Another brief element is the repeated use of a certain phrase after someone or something dies in the book: "So it goes." This phrase, according to Billy, is the Trafalmadorian response to death. Obviously it is a casual phrase; combined with its frequent repetition and its penumbra of fatalism, it imperils the morality of the tale. But Vonnegut leavens the narrative with serious and potent moral claims.
I have told my sons that they are not under any circumstances to take part in massacres, and that the news of massacres of enemies is not to fill them with satisfaction or glee. I have also told them not to work for companies which make massacre machinery, and to express contempt for people who think we need machinery like that. (Chapter 1)
Slaughterhouse-Five is not a fervent novel, but neither is indifferent. The ambiguity of plot and hero make definite judgment difficult, but "humanist" seems right.