Fanfictions that hate Dumbledore often deviate from the canon in how they portray his character's intentions. In the canon, Dumbledore is generally seen as a wise and benevolent figure, albeit with some flaws. But in these fanfics, his actions are often painted as more self - serving. For example, his actions towards Harry might be shown as being more about his own legacy or power plays rather than protecting the wizarding world as in the original story.
Some fanfictions portray Dumbledore negatively because he made many controversial decisions in the original series. For example, his handling of Harry Potter's situation was sometimes seen as using Harry as a pawn in the fight against Voldemort. He kept many secrets from Harry that could have potentially changed the course of events and put Harry in more danger. Also, his relationship with Grindelwald in his youth and the events surrounding it add to the reasons why some writers in fanfics might choose to show him in a negative light.
It could be that Harry finds out about Dumbledore's secrets regarding the Deathly Hallows. If Harry thinks Dumbledore was more interested in the Hallows for his own purposes rather than for the greater good, it might make him hate Dumbledore.
In some fanfics, Harry might hate Dumbledore because he feels Dumbledore has been using him all along as a tool to defeat Voldemort. Dumbledore has been making decisions for Harry without fully considering Harry's feelings and desires. For example, he didn't tell Harry about the prophecy until it was almost too late, and Harry might see this as a form of manipulation.
In some fanfictions, female Harry Potter might hate Dumbledore because the author wants to create a different dynamic. Maybe Dumbledore's actions in the original story, like keeping secrets from Harry, are exaggerated in the fanfic. For example, if he withheld information that could have saved someone she cared about, it could lead to her hatred.
In some fanfics, Dumbledore having 'babies' is a very strange and often out - of - character concept. Usually, if this idea is explored, it might be in an alternate universe kind of setting. Some fanfics may use it as a shock factor or to create a really different kind of family dynamic within the wizarding world. But it's not a common or canon - like concept at all.
In the 'Harry Potter' fandom, this would be a very controversial concept. Dumbledore is often seen as a mentor and a powerful, positive figure. Having Harry Potter destroy him goes against the general perception of their relationship in the original series. However, in the realm of fanfic, some might explore such a dark and alternative take as a way to examine different character arcs and what - if scenarios, but it would likely be met with a lot of resistance from purist fans.
Well, it could be that Harry discovers some dark secrets about Dumbledore's past actions. Maybe Dumbledore made a deal with some dark forces long ago that Harry thinks endangers the wizarding world. So, in a fit of moral outrage, Harry decides to 'sack' him, which could mean removing him from his position of power and influence at Hogwarts.
Maybe he hates it because it often distorts the real story or characters he knows. Fanfics can take the story in wild directions that don't align with his own experiences.
In some fanfics, Harry might be made to hate Gryffindor perhaps because the author wants to explore an alternative personality for him. Maybe they want to show how the Gryffindor house's values, like bravado or recklessness, could be seen in a negative light from Harry's perspective.