Vonnegut wasn't "a nihilist" (not... many people are, really? Approximately zero? Not in some all-encompassing sense, anyway—"I'm a nihilist" usually means subs

Vonnegut wasn't "a nihilist" (not... many people are, really? Approximately zero? Not in some all-encompassing sense, anyway—"I'm a nihilist" usually means subscribing to some philosophy that features nihilistic elements in some parts of it, or to having accepted, specifically, existential nihilism as a ground truth for one's further exploration of philosophy) but definitely expresses elements of existential nihilism in his writing, which is what people usually mean when they say stuff like "I'm a nihilist" or "so-and-so is a nihilist": rejection of (the possibility of) objective meaning or purpose. Nihilism's a feature of positions/schools more than some philosophical school of its own, and that feature's strongly present in much of Vonnegut's work. Existential nihilism is a key element of absurdism (which you mention)—in a philosophical sense, and in the places where that intersects with literature, not necessarily in the colloquial senses of the word. In some areas, sure, Vonnegut's not particularly nihilistic. I don't think it'd make any sense to associate him with moral nihilism, for instance. Watts, IDK, he's on my to-read list but it's a looooong list.