The humanness of a villain can certainly be a weakness, but it can also be that which makes the villain truly terrifying. They might realize that acts they’re committing are wrong, but they usually claim that “the ends justifies the means.” It’s this twisted justification that creates the fear; if you find yourself agreeing with, or even understanding, your villain’s line of reasoning, make it more arbitrary or make the leaps of logic farther and less straightforward.

People who commit evil acts, recognize that the acts are evil, and have no justification or reasoning for them are psychopaths. While psychopaths occasionally make great villains (e.g. The Joker), they are often difficult to write for and usually seem too unrealistic.