The least interesting thing an antagonist can be is correct about nothing. I don't mean antagonists need to be right — Samuel Carrington in Shadow of a Witch is about as far from a sympathetic figure as I've written — but he has to believe, coherently and completely, that his version of events justifies what he does next. The moment a villain exists purely to be defeated, a reader stops fearing them and starts merely waiting for them.
Some of my antagonists aren't people at all. Across the Legions of Honour series, the real threat Gaius faces usually isn't a single Roman officer — it's Rome itself: patient, bureaucratic, entirely convinced of its own righteousness, and for that reason far harder to out-manoeuvre than a sword. An institution that has decided it's right doesn't need to be cruel to be terrifying. It just needs to keep showing up, correctly filed and perfectly justified, until it wins by sheer administrative persistence. That's the antagonist at the heart of Measured Ground, and it frightened me more to write than any single villain has.
Then there are the antagonists closer to the ground — Lenny Blake in Hidden Cargo, for instance, who isn't a monster in his own mind at all. He's a businessman managing risk, and the fact that the risk is human beings is, to him, an inconvenience of the trade rather than the point of it. That gap between how a character understands themselves and how a reader understands them is usually where the real dread lives — not in what the villain does, but in how unbothered they are while doing it.
Years spent investigating people who'd done genuinely terrible things taught me the same lesson fiction eventually did: almost nobody thinks of themselves as the villain of their own story. Write from inside that conviction, however briefly, and the antagonist stops being a plot obstacle and starts being the reason the book is hard to put down.