I’m a big fan of Nabokov and his despicable characters. Love them, love to hate them, love to get under their skin and understand what makes them tick.
I have written some despicable characters too, serial killers and proper weirdos, and I try to make the reader love to get to know them like I loved getting to know Humbert Humbert in Lolita. I‘m a big fan of an unreliable narrator, and a story that doesn’t play by the rules.
It’s such fun to imagine the thought processes that make up a truly awful person, to figure out how someone turned out that way, to backtrack to that moment that turned them, and that’s the trick to creating such a person
Every character should have a back story, a pivotal moment in their childhood or previous life that made them who they are before they even feature in your story. Especially the villains. They have to have reasons why they do things, anger about something, a murderous motives, or a warped view of the world.
Make your characters rounded with real life experiences, trauma, emotional scarring. There had to be a moment they remember changing, a moment that stuck with them, a moment that made them who they are, a moment that made them do what they do. Then they’ll be believable.