Commissioner Gordon might also take on the role of the main antagonist. He has always been an ally of Batman, but if Batman goes evil, Gordon would have to use all his resources and his understanding of Gotham's underworld to try and stop the Dark Knight. He could form a special task force dedicated to taking down the now - evil Batman.
The Bat - family members like Robin or Nightwing could become the main antagonists. They would be horrified by Batman's transformation and would try to stop him from causing more harm. Their knowledge of Batman's tactics and his lair would be their advantage in trying to bring him back to the side of good.
Another possibility is that a new hero or anti - hero could emerge as the main antagonist. Someone who sees the threat that an evil Batman poses to Gotham and decides to take matters into their own hands. This could be a character who has been in the shadows of Gotham all along and now steps into the spotlight to face the corrupted Dark Knight.