It could be that in the fanfic, Harry discovers some dark secrets about Dumbledore's past. Maybe Dumbledore was involved in something that goes against Harry's sense of justice. For instance, if Dumbledore had some connection to a dark magic event that Harry finds out about, it would make Harry lose his trust and start to hate him.
Perhaps in the fanfic, Dumbledore's actions towards Harry's friends or family led to some bad consequences. If Dumbledore made a decision that got one of Harry's friends seriously injured or in danger, Harry would naturally be angry at him and that anger could turn into hatred.
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.
Perhaps Harry has his own vision of Gryffindor and the fanfics he's seen don't align with it. He might have a deep - seated understanding of what Gryffindor means to him, like courage, loyalty, and friendship in a very personal way. The fanfics, on the other hand, might focus on more superficial or inappropriate aspects of Gryffindor for his taste, such as focusing too much on power plays or false bravado rather than the true essence of what makes Gryffindor great.
It could be that in the fanfic's plot, McGonagall has some past connection to Harry's enemies. So, her hatred towards Harry is a result of her loyalty to those she has a history with. This would create a lot of drama and conflict within the fanfic's story.
Well, it could be that he has seen some really bad magic fanfic. Some fanfic might be full of inaccuracies about magic, and as someone who has a deep connection to magic, it annoys him. For example, if a fanfic misrepresents a spell or a magical creature that he is familiar with, it would make him dislike it.
Maybe in the fanfic, Harry has his own ideas and beliefs that conflict with Dumbledore's. He might think Dumbledore's plans are too risky or not in line with his own sense of justice.
Maybe Harry has a major disagreement with Dumbledore's decisions or strategies and thinks a duel is the only way to settle it.
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.
It could be due to a sense of abandonment. Fanfic writers might explore the idea that Harry, growing up without his parents, feels resentful. He may see other kids with their families and, in his confusion and pain, start to hate his parents for leaving him alone in the world, even though it was not their choice.
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.