What to do with Antagonists (in general)?

Every hero story has a bad guy in it. The villain.

Harry Potter vs Lord Voldemort
Frodo Baggins vs Sauron
Luke Skywalker vs Darth Vader...you get the idea.

Watching them from afar is okay. You're not the writer, you don't need to worry about how the villain plays a role in the MC's plotline.

But when you're the writer, you have to think, who are my bad guys? Or,