Antagonist

Antagonist; The antagonist is the primary force or person working against the protagonist. These terms can not be defined as "Bad Guy" or "Good Guy", as it could work either way depending on who fills what role. For example, in a Mario game, Mario and Luigi are usually the protagonists, while Bowser is the antagonist. However, if you played the game as Bowser, even though he'd be the Villain still, he would also now be the protagonist, while Mario and Luigi, still heroes, would now be the antagonists.

The same could be said about many other groups or characters.

In the manga and anime series, Death Note, there is a man named Light, who is the protagonist, who is killing criminals, because he wants to rid the world of crime and become its god. To accomplish this, he uses his Death Note, a magical note book capable of ending human lives which once belonged to a god of death. The Antagonist would be considered the police, because they police are trying to stop Light from killing anyone else. By this book, the antagonist doesn't always have to be the bad guy.