To be villainous is to be evil, wicked, and despicable, like a bad guy in a movie or a villainous bus driver who sees you but doesn’t stop.
Villains in stories are the bad guys, like Superman's enemy Lex Luthor or a mobster in a crime movie. Real-life bad people are also called villains, and anything evil is villainous. To murder someone is villainous. To steal a lot of money is villainous. This is a strong word for actions that are deliberately harmful and awful. You can’t be villainous by accident. The opposite of villainous is heroic.