Every TV show needs a hero or two for the audience to invest in, and it's equally true that every great hero needs a villain to go up against.
This rule has led to some of the greatest rivalries in television history: Buffy vs The Mayor, Cooper vs BOB, Daredevil vs Kingpin... Tom vs Jerry, etc etc and so on. These clashes of good guys and Big Bads, of immovable forces and unstoppable objects, give their series much of their interest and drive, both in terms of plot and character.
Where would Game of Thrones be without the likes of Joffrey Baratheon or Ramsay Bolton to root against? How would Lost have fared without Benjamin, or even The Simpsons without Mr Burns? We need someone to love to hate, and for the hero to win against. But just like how sometimes the best heroes are former villains, so too can a heel turn by one of the protagonists make for something truly compelling.
For a good person to flip the switch and irrevocably change who they are is not to be taken lightly, but when done right it can make for a good bad guy. It's one thing to simply start out evil, but even more interesting to be pushed in that direction.