One reason could be that he doesn't like how the fanfiction writers change the storylines. For instance, they might create romantic relationships for him that he doesn't approve of. It's like his own story is being rewritten without his consent.
Perhaps he thinks that the fanfiction dilutes the real essence of Camp Half - Blood. The real place has a certain magic, danger, and camaraderie that he fears might be lost in the fictionalized versions. He wants people to understand and respect the real Camp Half - Blood as it is, not through some made - up stories that might mislead others.
He may also be tired of seeing so many different versions of himself in fanfiction. Percy has his own identity, and when he sees all these different interpretations in fanfiction, it might make him feel like his true self is being overshadowed. Also, some fanfictions might contain inaccuracies about the Camp's rules or the abilities of the characters, which he, as someone who has lived through those things, would find irritating.
One possible reason could be a dark turn in his character. Maybe he's been corrupted by some external force like a powerful and evil magic. Another reason might be a misunderstanding. For example, he believes the camp has betrayed him in some way, so he lashes out in anger.
One possible reason could be jealousy. Maybe Percy gets more attention or recognition in their fictional world that makes Annabeth envious and thus appear to hate him.
One possible reason could be differences in their divine natures. Percy is a son of Poseidon, a god associated with the sea and a more fluid, emotional element. Artemis is a virgin goddess of the hunt, very strict and unyielding in her ways. This contrast in their essences might lead to clashes and Percy's dislike.
He might leave because of a personal conflict. Perhaps he has a falling - out with some of the other campers or with the camp leaders. Like if he disagrees with a new rule or policy that the camp has implemented which he thinks is unjust.
One reason could be that he doesn't like how some fanfiction writers misinterpret his character. They might make him act in ways that are completely out of character in relation to Renesmee. He has a certain image of himself and his relationship with her, and fanfiction can mess that up.
One reason could be that he was treated unfairly by other shinobi. If he was constantly looked down upon or discriminated against by his fellow shinobi, it might make him develop a hatred for them.
It's possible that the writer of the fanfiction has given Akeno a backstory or personality trait that directly clashes with Naruto's. Maybe she is extremely arrogant and Naruto, who is humble and believes in everyone's potential, would find that detestable.
It might be due to the way their goals are presented in fanfiction. Naruto is often about protecting his village and friends. If Artemis's goals in the fanfic are in conflict with Naruto's, like if she's more focused on her own self - interest in the story, Naruto would hate her.
One possible reason is that Naruto is so focused on his ninja training and his goals in the shinobi world that he views Christmas as a distraction. He might think it takes people's attention away from more important things like self - improvement and protecting the village.
One reason could be a conflict of interests. If they are competing for the same thing, like a magical artifact or a position in a magical society, and Susan uses underhanded tactics, Harry would hate her.