Being a "humanist" and believing in God are not mutually exclusive. You should go read what he wrote about that.
Interviewer: "Did the study of anthropology later color your writings?"
Vonnegut: "It confirmed my atheism, which was the faith of my fathers anyway. Religions were exhibited and studied as the Rube Goldberg inventions I'd always thought they were."
Atheism and humanism are the equivalent of the letters Q and U which rarely appear, in English, one without the other; further, both philosophies can be reduced to worship of self.
Vonnegut was a documented atheist/humanist and socialist. Now, were I to compare letter usage in the French language, the letters Q, U and E would be an exact representation of the trio.
Worse, Vonnegut admires jihadist suicide-bombers:
In an interview with an Australian newspaper, Vonnegut made the comment that suicide bombers are "very brave people". Contrary to the common assertion that suicide bombers act simply because they hate freedom, Vonnegut described their motivation as being that "They are dying for their own self-respect. It's a terrible thing to deprive someone of their self-respect. It's [like] your culture is nothing, your race is nothing, you're nothing." He concluded by saying that "It is sweet and noble -- sweet and honourable I guess it is -- to die for what you believe in."
Yep, he's insane.