Batman would be heartbroken. He has trained Robin and sees him as a partner and almost like a son. He would first try to talk to Robin, to find out what made him turn evil. He would hope that he could reason with Robin and bring him back to the good side.
Another plot could be that he is exposed to some sort of mind - controlling substance or technology. The enemy could be using it to turn him against Gotham. Under this influence, he starts to see his friends as foes. He could begin to attack the Bat - family and other superhero allies. His actions would be erratic and violent, and the city would be in chaos as they try to figure out what happened to their hero and how to stop him.
One possible storyline could be that Batman is exposed to a mind - controlling substance. This makes him turn against his allies in Gotham. He might start using his advanced technology and combat skills to take over the criminal underworld instead of fighting it.
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.
The Joker would probably be both delighted and a bit confused. He's used to Batman being his nemesis, but an evil Batman would change the dynamic. He might initially see it as a new and exciting game, but then realize that an evil Batman could be a real threat to his own chaotic existence. The Joker might try to manipulate the situation to his advantage, but it could easily backfire on him.
Another aspect could be the formation of new alliances. Since Superman is evil, Batman might have to team up with other unexpected characters. For example, he could join forces with Lex Luthor, who usually opposes Superman. This would create a very different power dynamic and storyline, as they have to put aside their differences to face the greater threat of an evil Superman.
One popular 'evil superman x batman fanfic' might be those where Evil Superman tries to manipulate Batman into joining his dark plans. For example, in some stories, he uses his super - powers to put pressure on Gotham, forcing Batman to make difficult choices between saving the city and standing against Superman's tyranny.
One possible plot twist could be that Robin is influenced by a powerful and evil villain. Maybe the villain uses some kind of mind - control device or magic on Robin. For example, the Joker could find a way to make Robin see Batman as the enemy. Robin might then start attacking Batman and helping the Joker with his evil plans.
Kids' stories often show Batman as the symbol of good. He fights crime and protects the innocent. The villains in the series, like the Penguin or Catwoman, are the bad guys. So kids see the clear divide between good and evil through these characters.
In this genre, the relationship is often about conflict. Evil Superman challenges Batman's moral code. Batman, being a symbol of justice in Gotham, is pitted against a super - powered being who has turned to the dark side. So, it's a clash of ideologies.