Schopenhauer believed that will — the blind, striving force beneath all conscious action — is more fundamental than reason. We don't decide to want things; wanting is what we are. A science fiction thriller can steal this premise whole: your most terrifying antagonist isn't the one with the superior plan, but the one whose desire has no object. Not power, not survival, not revenge — just the pure forward pressure of a will that has outlasted every goal it was ever attached to. The horror isn't malevolence. It's purposiveness without purpose, an engine that can't stop because stopping would require a reason, and the reasons have all burned off. Your protagonist then faces something that rational negotiation, moral appeal, and even sacrifice cannot touch — because those moves assume an interlocutor who wants something specific. When you write the scene where your hero finally offers the antagonist everything they asked for and the threat doesn't diminish, you've found the structural nerve of the book.

What are you currently working toward that, if you achieved it tomorrow, you suspect would not actually stop the striving?

Drawing from German Idealism / Pessimist Philosophy — Arthur Schopenhauer (The World as Will and Representation, 1818)