I find antagonists need to be more human and focused than anyone else in the book. Although, not necessarily complex.
What I mean, most of the time the antagonist is pushing an agenda against the main character’s.
So I’d write them as another human being driven by a purpose or event. I’d make them as important as the protagonist.
Sometimes, I reread my story swapping the protagonist and antagonist around. If the antagonist was in this situation (rather than the protag), how would they play it out? What would be their feelings and take-out?
Hope that helps a bit.