Well, it could be because in the original books, there are certain plot points where Dumbledore's actions seem a bit shady. For example, his relationship with Grindelwald in his youth. Some fans take this and run with it, thinking that maybe Dumbledore wasn't as good as he was made out to be. So they write fanfictions to show a different side of him, where he's not the all - wise and benevolent headmaster. They might explore how his decisions affected the other characters, like Harry, in a more negative way than was initially presented.
Some fans might feel that Dumbledore was too manipulative in the original series. They create these fanfictions to explore an alternative view where his actions are not excused.
There are those who think that Dumbledore's treatment of certain characters, like Snape, was unfair. In fanfictions, they bash Dumbledore by highlighting these perceived injustices. They may also feel that his overall plan for Harry was too convoluted and put Harry at unnecessary risk. By creating these 'Dumbledore bashing' fanfictions, they are able to rewrite the story in a way that holds Dumbledore accountable for what they see as his flaws.
Some people write these fanfics because they feel Dumbledore had too much control in the original story. They see Lord Potter as a powerful character who could have made different choices without Dumbledore's interference.
Well, in some fanfictions, Dumbledore is bashed because writers want to explore alternative storylines. They might believe that Dumbledore was too controlling. He made a lot of decisions for Harry without really consulting him. So by bashing him, they can create scenarios where other characters step up or Harry has a different kind of growth without Dumbledore's influence.
It could be that they simply want to explore different character dynamics. In the original books, Dumbledore is a very powerful and respected figure. By bashing him in fanfiction, they can create new power balances and relationships within the Harry Potter universe. This allows for a fresh take on the story and can lead to interesting character development that wasn't possible in the original story.
Some fanfictions focus on Dumbledore's mistakes in raising Harry. They might show that his distance and secrecy were not just for protection but rather a sign of his lack of true care. For example, in a particular fanfic, Dumbledore is blamed for not being more forthcoming with information about Harry's parents and the prophecy, which leads to Harry making many mistakes and facing more danger than necessary.
Some fans write it because they feel Dumbledore has some controversial aspects that the original books didn't fully explore. They want to dig deeper into his character and show the darker side that they think exists.
Another reason could be that they simply want to create a different narrative. Maybe they love the Harry Potter universe but feel that Dumbledore was too much of an all - knowing, all - powerful figure. By 'bashing' him in fanfic, they can introduce new plotlines and character developments that deviate from the original and offer a fresh take on the story.
There are fanfictions where Dumbledore is bashed for his secrecy. For instance, his withholding of information from Harry about his family and his connection to Voldemort. Some female - authored fanfictions might focus on how this secrecy put Harry in more danger than necessary. They might rewrite the story from a perspective where a more open Dumbledore could have changed the course of events for the better.
Some exist because some fans really ship Harry and Hermione (HHR) and feel that the Weasleys, especially Ron, are in the way of their ideal pairing. So they create fanfics to bash the Weasleys to make their preferred relationship seem more likely.
Some exist because fans might feel that the concept of time travel in the Harry Potter series was not fully developed. They use bashing fanfiction to explore the flaws. For example, in the books, time travel seems a bit too convenient at times.
Some fanfictions like 'Harry Potter hides as a girl' exist because it's an interesting 'what - if' scenario. It allows authors to explore different aspects of the Harry Potter universe in a new light. They can play with the character dynamics and how the story would change if Harry had this different identity.