Is there a moral in the Slaughterhouse Five?
Concerning Slaughter-House Five, I'll answer your question by dealing with two fundamentals of your thinking.
First, this novel is not an Aesop's Fable--don't expect a one-line moral. Most sophisticated fiction raises issues and reveals themes, rather than presenting nice, neat, easy one-liners that tell a reader how to live. You raise the issue of the postmodern writer. Most postmodern writers know better than to think they have all of the answers: the world is a complex place and postmodern fiction reflects that. Life can't be boiled down to one-liners.
Second, you mention that as a postmodernist writer Vonnegut isn't supposed to influence the reader's thoughts. Your idea here needs to be refined. Some postmodern writers stress that all sides of an issue should be fairly treated in a novel. This again reflects the lack of absolutes in existence and the complexity of existence. The writers know better than to think there's only one side to every story. But this isn't the same as not influencing the reader's thoughts. A writer that cannot in any way influence a reader probably wouldn't bother writing. Postmodern writers seek to influence readers, they just don't do it didactically like a sermon, and they reflect the chaos or ambiguity (different interpretations) of the world.
The term, postmodern, however, covers a great deal of literary territory. Vonnegut, for instance, is a satirist. As such, postmodernist or not, he attacks targets in his writings. Vonnegut often does not attempt to present all sides of an issue. He attacks targets. In this novel, war is dehumanizing and destructive; human beings are capable of great cruelty; human beings are shortsighted and don't understand consequences of their actions (that's what the time games played in the novel are about, not fate and free will), etc.
By the way, as a side note, knowing what will happen doesn't take the meaning out of it--that's faulty logic. The meaning remains whether Billy knows it's coming or not. The knowing ahead of time plays into another device Vonnegut uses: the detached narrator. The detached narrator is not Vonnegut not knowing how to get his thoughts wrapped around the events. The detached narrator is a literary device used to create an ironic tone and understatement. The understatement forces the reader to create the horror in the events, rather than the writer having to do it, which is extremely difficult.
In other words, rather than the narrator trying to convince the reader how horrible an event is (which a reader may resist) understatement moves the reader to create the horror and the meaning. The reader discovers and creates the meaning, rather than having it shoved at him or her. The narrator is a literary device, a sign of Vonnegut's ability as a writer and thinker, not his inability.
I would actually argue that the moral isn't necessarily about free will or the lack thereof. When I read the book, I felt that Vonnegut addressed things with the uncaring attitude because some things are almost (if not completely) impossible to wrap our heads around. Having witnessed the horrific carnage that was the result of the Allies fire-bombing Dresden and burning tens of thousands of civilians alive, how would Vonnegut come back and talk about the war in a country where they felt like they had "won" the war and saved the world?
In this way, the approach of the Tralfamadorians makes sense in a way, they know terrible things will happen, when someone dies it is simply the way it is, no sense in trying to avoid it or get emotional about it.
So I thought the moral was, in some sense, similar to that of Lord of the Flies or other books that have addressed the great capacity for evil within man, even men that consider themselves civilized.