Batman would likely be very cautious. He knows Robin's skills well, so he would approach any confrontation carefully. He might start by observing Robin's actions from a distance to understand his motives better. At the same time, he would try to figure out if there is an external force influencing Robin, like a new villain or some kind of mind - control.
Batman would be in a difficult situation. On one hand, he doesn't want to hurt Robin. But on the other hand, he has to stop Robin from causing more harm. He might try to isolate Robin from any external bad influences. He could also try to remind Robin of their past together, the good they've done, and the values they once shared in an attempt to make Robin see the error of his ways.
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.
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.
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.
At first, Batman would be shocked and a bit confused. But quickly, his detective instincts would kick in. He would view the deaged Joker as a new and unexpected threat. He'd start gathering information about how the deaging occurred, in case it was part of a larger, more sinister plan. Batman would also be very careful not to underestimate the deaged Joker. He might try to isolate the Joker in a secure location to prevent him from causing chaos in Gotham. And he'd be constantly on the lookout for any signs of the Joker's old, devious self emerging from this new, younger form.
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.
Batman would probably isolate himself at first. He'd retreat to the Batcave to analyze the situation. He might feel a sense of guilt, thinking he should have seen the betrayal coming. After that, he'd start planning his counter - move. He'd use his vast resources and knowledge to expose the truth behind the betrayal and take down whoever is behind it.
The Justice League as a whole would have a diverse range of reactions. Some members like the Flash would be all for it, seeing it as a fun exploration of Batman. Others like Wonder Woman might use it as a way to understand how the public perceives Batman's role in the League. And then there are those like Aquaman who might just shrug it off unless it directly affects his interests. The fanfiction could also potentially create some internal discussions within the League about how they are all portrayed in the public's imagination.
Batman might become more brooding. He could withdraw from the people around him, like Robin or Commissioner Gordon.
She might be very surprised at first. After all, Batman is not one to show his emotions so openly. But then she would likely be overjoyed. She has deep feelings for Batman too, and this proposal would be a confirmation of their complex relationship.
Some fans might be really excited about it. They love to see new and unexpected relationships explored in fanfiction. It gives a fresh take on the characters and their dynamics.
Some might be shocked. For instance, Batman is often portrayed in very different ways in fanfiction compared to his 'canon' self. Superman might raise an eyebrow at a fanfic where Batman is overly emotional. Others like the Flash might be really into it, finding the different takes on Batman's character and his adventures exciting.