Ah, the old fate and free will discussion: we meet again. And we'll probably be meeting a few more times down the road. Writers just love this topic, and truthfully, so do we. Cat's Cradle draws a really fine line between fate and free will. John thinks the path he takes in the novel was determined by fate. After all, The Book of Bokonon says things happen as they should. On the other hand, the novel doesn't give any evidence beyond John's assurances that everything that happened wasn't just a huge coincidence. Also, Bokonon is a liar and a half. So, is fate another of the lies humans tell themselves to make their lives bearable? Alternatively, is there a force guiding us to some predetermined fate against our free will?
John only believes in fate after the novel's conclusion. This means all the references to his fate come from future John—aka John the narrator—and not his past self. Like an M. Night Shyamalan movie, you'll have to reread it to catch all the clues.
The novel suggests that inventing technology guarantees its use. Don't want to see the world blow up? Don't invent something that can explode.