Perhaps in the fanfic, he's under the influence of some sort of mind - controlling substance or magic. This external force could be driving him to harm himself as part of a larger, more sinister plot against him.
One possible reason could be extreme self - blame. Batman might cut himself as a form of self - punishment for his failures in protecting Gotham. For example, if a major villain caused great harm despite his efforts, he could turn that guilt inward.
One reason could be that Batman is written as being too self - absorbed in the fanfic. He might be so focused on his own mission that he doesn't want the distraction or responsibility of taking in Robin.
Batman might avoid Wonder Woman in fanfic because he's afraid of getting too close to someone with such strong emotions. He's used to suppressing his own feelings, and Wonder Woman is very passionate. Her presence could stir up emotions in him that he doesn't want to deal with in the fictional stories.
One reason could be that Bruce is being too reckless in the fanfic. Alfred has always been there to keep Bruce safe and if Bruce is constantly putting himself in unnecessary danger as Batman, Alfred would be disappointed. For instance, if he jumps into a fight without proper preparation or a well - thought - out strategy.
One reason could be to explore a darker side of their relationship. Maybe the writer wants to show that even heroes can have moments of extreme frustration or anger towards their sidekicks. It could also be a way to shock the readers and make them think about the characters in a new and unexpected way.
It could be due to external factors in the fanfic's universe. There might be some political or social unrest in Gotham that makes Batman hesitant to take in a young sidekick. Or perhaps there are other heroes or vigilantes who are vying for Dick Grayson's attention and training, so he doesn't end up with Batman. Additionally, the writer might be exploring a darker version of Batman who doesn't believe in having a sidekick at all.
The fanfic might use this self - harm as a symbol of his internal struggle. He's constantly fighting crime, but at the same time, he's fighting his own demons. The act of cutting himself could be a physical manifestation of that internal battle between his sense of justice and his own psychological issues.
One reason could be the inaccurate characterizations. Fanfiction writers might not fully understand the nuances of Nightwing or Batman's personalities, and Nightwing hates seeing a wrong version of them. For instance, if Batman is made too soft or too harsh in a way that doesn't match the established canon.
One possible reason could be mind control. Maybe some villain has taken over Superman's mind and made him do it. Another reason might be a fundamental difference in their ideologies that has escalated to the point of no return.
It might be related to financial aspects. If the sales of the graphic novel have been consistently low, the publishers may decide to end it. Another possibility is that the creative team has had internal disagreements and they can't continue working together on the project. This could lead to the decision to end the Batman graphic novel. Also, sometimes a series is meant to be a limited run from the start, and 'to end on' could just be the natural conclusion of that planned run.
One reason could be her power over plants. She can control them to attack Batman, like making vines wrap around him.