That's my favorite kind of antagonists - the kind that are written realistically. There's no such thing as the Disney kind of evil, all people are capable of evil. But actually doing the evil deeds is what makes you evil, and though there are always serial killers and sadists, the best villains are the kinds written to believe in their motives. They need to be relatable, as in, they do what they do because they think their vision of the world would work.