Back when Pulp magazines were popular, readers expected the villains to cackle and gloat about their evil schemes, while tying helpless victims to railroad tracks, over the loud blare of an oncoming train. Now, that trend has been used so many times , its just a boring cliche of a non-realistic villain.
What makes a realistic villain?
If you learn about evil men from over the centuries, they all had a reason why they did what they did. It wasn’t because the guy or gal just was plain evil. No wonder how silly or crazy the reason is, villains have a motivation. Just like heroes. All heroes have some motivation, and are never just heroic to be heroic. Frodo went on his quest to destroy the One Ring to save The Shire. Peter Parker becomes Spider-Man to stop evil from happening to more people.
Even schoolyard bullies from Elementary Schools have motivations. It can be simple as they want to play with your ball, and they’ll take it from you. And it can be as complex as them wanting your milk money so they can buy milk for their cat. .
What I do is research some of the most evil people from history and figure out motivations to get ideas. (And always remember that a villain thinks what they do is the right thing, like Hitler and Manson. Even the most evil person is a person.) I’ve even done that with people I’ve known. (If you do that, do not the villain in your fictional story after them. That’s a lawsuit waiting to happen.)
The best piece of advice I can give is to take a villain stereotype, throw it on it’s head and make the villain your own. Its tough to do, but it is integral if you want your book and characters to seem fresh.
We already have enough Darth Vaders.