That's the thing about characters: they are, by default, more interesting when they're human. I'm not necessarily talking about biology....I mean human in terms of psyche, emotions, and depth. Humans aren't simple or one-dimensional. We are multi-faceted, with varying degrees of good and bad displayed at any one time.
When I go about creating my antagonists, the one thing I try to remember is love. Love is a universal emotion. Everybody wants to love someone, and to be loved in return....I'd go as far as to say that love is the most powerful and primal human desire. Even "bad" people like Mao and Hitler desire affection. After all, where would we be without human contact?
I try to remember this whenever I write a villainous character. Who do they love? Have they suffered the loss of a loved one? Who loves them? What was their childhood like? Thinking about questions such as these helps me create more multi-dimensional antagonists. Although I love the Harry Potter books, I don't find Voldemort to be a particularly interesting villain because he is "evil" from the start. Even as a small child there isn't an ounce of love or compassion inside him, and he doesn't care about receiving affection from others. It just isn't realistic. Tom Riddle was once a person, after all. This is one aspect of the Harry Potter universe that I feel JK Rowling left unexplored....what makes a person evil? Why does Voldemort commit such atrocious acts of violence? You cannot paint the world in shades of black and white. All people are capable of horrible deeds, just as all people are capable of good deeds, and an antagonist should have a reason behind their so-called "evilness".
Remember, none of the world's great "villains" thought of themselves as evil. I think we can all agree that Hitler did terrible, terrible things, and probably suffered from pretty severe mental illness, but I guarantee you he didn't hide out in a dark room going "MUAHAHAHAHA MY EVIL SCHEME IS WORKING." Misguided and delusional as he might've been, Hitler actually thought ethnic cleansing was the answer to the world's problems. When I write antagonists, I ask myself, "What does this person see when they look in the mirror?" How is their perspective different from the protagonist's? Do they believe themselves to be acting for the common good?
Depth is the key to a good villain. Strong protagonists are essential to any novel, but your antagonist should be equally developed. This will create a balance that enhances every aspect of the story.