Storytellers must be very good people, for they often love evil antagonists. A good antagonist is "complex, multi-dimensional and fully human," Laurie Hulzer says in her e-book, "How to Evaluate Stories."
They are driven by deeply personal needs and fears. They were wronged in the past. Something they prized was lost or stolen. A beloved parent or teacher left them. Because they nurse an unhealed wound, they are willing to "sacrifice empathy, ethics, honor, integrity, generosity, real relationships, or other human values like justice, mercy, compassion, or forgiveness."
It's not too awfully hard to imagine how an evil person might act. I have to think only of religious terrorists or mass murderers. They have reasons for the heinous things they do. Their justifications may be twisted or warped, but I suspect many of them act out of fear.
A man abandoned as a child may fear there is no one he can ever truly trust. He may fear he is unlovable (why else would he have been abandoned) and protect himself by never loving anyone.
She grew up impoverished, suffering greatly, and vowed never to be poor again. Stealing, murder, assuming another's identity--all means are justifiable to meet the inner need for security.
In my WIP, the antagonist, when he was young, lost the woman he loved to his best friend. He became a prominent doctor, researcher and educator. He has everything a sane man could want, yet he is still crazy with jealousy.
His best friend was entrapped by the events of one horrific night and is incarcerated in a mental hospital. The antagonist is his doctor. He has complete power over his best friend and wields it under a cover of concern and compassion. But his fears have turned him into a shell of a human being. He is incapable of loving his mousy wife. He sleeps with his female students. He lies. He is consumed by the needs of his past and by his fears of being exposed as a loser. He will do everything to bury the truth.
Not every story has--or needs--an evil antagonist. But I often like the ones that do better.