To me, 'ruthless' nails it best. It carries a quiet, efficient cruelty that doesn’t need theatrics — the villain who trims empathy away and treats people as obstacles. 'Ruthless' implies a cold practicality: they’ll burn whatever or whoever stands in their path without hesitation because it serves a goal. That kind of language fits manipulators, conquerors, and schemers who make calculated choices rather than lashing out in chaotic anger.
I like using 'ruthless' when I want the reader to picture a villain who’s terrifying precisely because they’re controlled. It's different from 'sadistic' (which implies they enjoy the pain) or 'brutal' (which suggests violence for its own sake). For me, 'ruthless' evokes strategies, quiet threats, and a chill that lingers after the scene ends — the kind that still gives me goosebumps when I think about it.