Everett's daughter reportedly committed suicide, believing she would meet Everett in a parallel world. Ideas about reality, whether they are true or not, can affect human behavior, even dangerously. If a legitimate scientist said something harmful, even if not intentionally, but as a hypothesis to explain something, people may take it seriously. Now I don’t know if I can go as far as to claim that Many Worlds is a dangerous idea, but I cannot rule it out as not dangerous either. As a scientific hypothesis, it’s consistent, but the danger is in how it’s received by people without a background in Physics and how it may affect their actions. 

This is what Vonnegut was talking about when he was writing about Kilgore Trout’s dangerous ideas, even when Trout meant no harm. The ideas we construe in science are not culturally sterile. 

Now. There are two kinds of dangerous ideas. On the one hand, there are ideas that actually correspond to reality and turn out to be dangerous. The nuclear arms race is an example. It started out as a scientific curiosity, then it became a technological curiosity, then patriotism, war, and now countries bullying each other. Other dangerous ideas don’t necessarily correspond to reality, but become psychologically as dangerous as if they were real. Everett’s daughter is an example.

I think on a societal level, the whole “the universe is indifferent and life has no intrinsic meaning and we have to create meaning” business of the mid twentieth century was a response to the possibly dangerous Laplacian worldview. Laplace isn’t right. We know that today. But we also don’t know what replaces Laplace. My own take is Wheeler's participatory realism, where novelty is constantly being created in the universe. There’s no reason to associate any religion with it. But I think it’s a more enthusiastic universe to be in. The universe still doesn’t care about our individual problems. But our actions in the universe contribute to the creation of novelty. You and I are co-authors in the creation.