Perhaps there has been a misunderstanding. Some false information has been spread within the Justice League about Batman, and he feels betrayed and decides to fight back. It could also be related to a new threat that Batman believes the Justice League is handling wrongly, and he tries to stop them in his own way.
One possible reason could be ideological differences. Batman has a very different approach to justice compared to some of the other members. He is more about the streets and dealing with crime on a very personal and gritty level. The Justice League often deals with larger - scale, more cosmic threats. Batman might feel that his methods are being overshadowed or not given enough consideration within the League. Another reason could be a personal crisis. Maybe something in his personal life, like a threat to his identity as Bruce Wayne or a new development in Gotham that requires his full attention, makes him decide to step away from the Justice League.
Perhaps Batman feels that he can better protect Gotham on his own. The Justice League has a wide - ranging mission across the world or even the universe. But Batman's main concern has always been his home city of Gotham. He might think that the time he spends with the Justice League is taking away from his ability to safeguard Gotham against its unique threats like the Joker and other Gotham - specific villains.
Another possibility in the fanfic is that Batman discovers some dark secrets within the Justice League. Maybe there's something corrupt or dangerous about the way they operate that he can't tolerate. So, to protect his own integrity and the safety of Gotham, he chooses to leave the team.
The plot could involve Batman saving the Justice League from a situation where they are physically trapped. Maybe they are in some sort of energy - based prison. Batman, with his detective skills, would first figure out how the prison works. Then, he'd use his gadgets like his grappling hook and batarangs to break through the prison's defenses and free his comrades.
Well, in this fanfic, perhaps the Justice League has been poisoned or mind - controlled. Batman, being the resourceful hero he is, might first analyze the situation carefully. He could then track down the source of the problem, whether it's a new super - villain or an old enemy with a new trick. He'd likely use his Bat - computer to find a cure or counter - measure and then fight his way through any minions to administer it to the League members, saving them from certain doom.
Batman is a master strategist. He would study the weaknesses of each Justice League member beforehand. For example, he knows that Superman has a weakness to Kryptonite. He would also use his advanced technology and gadgets to even the odds. His bat - suit is equipped with various tools that can be used against different opponents.
Maybe he feels betrayed. In fanfiction, there could be a situation where his fellow Justice League members keep secrets from him. For instance, Superman could be hiding something about his true powers or his origin that could potentially be dangerous. Batman, being the detective he is, values truth and transparency, and such secrets might make him feel that he can no longer trust his teammates and thus quit.
Perhaps Batman feels betrayed by his fellow members. A secret could be revealed about one of them that makes Batman lose faith in the integrity of the group. He's all about trust and if that's broken, he'd be likely to walk away.
One possible reason could be a misunderstanding. Maybe the Justice League misinterprets Batman's actions as being against them when in fact he's trying to protect them in his own way. For example, Batman might be keeping a secret that he thinks is for the greater good, but the League sees it as a betrayal of trust.
One reason could be a moral disagreement. Batman has a very strict moral code. If the Justice League were to engage in actions that he considered too extreme or unethical, he might quit. For example, if they started using excessive force on villains without proper trial. Another reason could be burnout. Constantly fighting crime and dealing with the politics within the Justice League could take a toll on him. He might feel that he needs to step away to regain his sanity and focus on protecting Gotham in his own way.