on Writers Helping Writers:

As readers, what inspires empathy for the hero and makes us root for him? Their flaws? Admirable qualities? Hopeless circumstances? Yes, to all of the above. But none of these elements would be effective without a worthy villain to complicate matters.

This is the real purpose of the antagonist: to make things unlivable for the hero and ramp up reader empathy. I mean, would we care so much about Snow White without the Queen? Maximus without Commodus? The Smurfs without Gargamel? Villains are important because they’re the ones who determine how bad things will get for the hero. It is fear of this antagonist that inspires empathy in readers, putting them firmly in the hero’s cheering section and ensuring they will keep turning pages. So it’s crucial you create a villain who is just as unique, interesting, and believable as the main character.

One way to do this is by including the Evil-By-Nature Villain.