How do writers come up with antagonists philosophies?

That’s part of the job of a writer. They have to devise all sorts of scenarios and envionments in which their plot will fit. A lot of it is imagination, probably with some research thrown in.

I remember reading about the science fiction author Robert Heinlein – famous for devising entire social structures for living on the moon. this was one of my favorites.

He was recognized not just for his science fiction acumen, but for having invented and devised complex social milieux for different groups on the moon, and how they all interacted.

Bottom line: a quality author spends a lot of time thinking before they every put pen to paper.

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Another author – I think it was Frank Herbert, who wrote Dune link(novel) did much the same in his work. But in addition to the story, I believe he also invented the grammar and syntax of an entire make-believe language as well.

Bottom line: It takes work!