A villain is a bad person — real or made up. In books, movies, current events, or history, the villain is the character who does mean, evil things on purpose.
Today a villain is a wicked person, whether in fact or fiction. In the 1300s, villain described a low-born rustic. It came from the Medieval Latin word villanus, or farmhand. Just why a word would evolve from meaning farmer into evildoer is a little mysterious, although it probably has to do with farmers not being chivalrous, like the knights who were so admired in those days.