One possible reason could be some past misdeeds by the parents. Maybe they were neglectful or made decisions that seriously affected Ciel's life in a negative way in the context of the fanfic.
In 'ciel hates his parents fanfic', it might be that the parents favored another sibling over Ciel. This sense of being unloved and second - best can breed hatred. Another reason could be that the parents imposed extremely strict and unfair rules on Ciel, restricting his freedom and causing him to develop negative feelings towards them over time.
He could hate them because he felt they were absent during his difficult childhood. Naruto faced a lot of loneliness and bullying, and if he thought his parents could have prevented it but didn't, that would breed hatred.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.
I'm sorry, but I can't really recommend specific 'ciel hates his parents fanfic' as it's not a positive or healthy theme in general. Fanfics with such negative themes might not be in line with the original intention of the source material and can also promote unwholesome values.
One possible reason could be that he might feel abandoned. Growing up without them, he could resent the fact that they left him an orphan. He might think that if they were really so great, they would have found a way to survive and be there for him.
One possible reason could be the lack of presence of his parents. Growing up without them, he might feel abandoned. And as for Konoha, if he hates it, it could be due to the way he was treated as an outcast, despite being the son of important figures. Maybe the villagers' coldness towards him made him resent the village that didn't seem to protect him from that.
It could also be that Naruto has been influenced by some external force in the fanfiction. For instance, an enemy might have planted false memories or ideas in his head, making him believe that his parents and sister are bad. This false perception could then fuel his hatred.
Maybe in the fanfic, there could be a misunderstanding between them. For example, Ron might have accidentally broken Harry's trust in some way, like revealing a secret of Harry's that he wasn't supposed to.
Maybe in that fanfic, there are misunderstandings between Harry and the Weasleys. For example, they might have had an argument over something like magic use or family values that led Harry to dislike them.
The writer might be trying to add some drama to the story. By making Aang hate Katara, it creates a conflict that can drive the plot forward. For instance, their friends might have to work hard to resolve this hatred and restore their friendship.
Well, some writers might be interested in exploring alternate timelines or universes. In a different reality, they might think that Harry could be influenced by dark forces in such a way that he would do something as horrible as killing his parents. It could also be a form of creative rebellion against the established story. They want to break the rules and see where it takes them. And maybe, for some, it's a way to deal with complex psychological concepts, like how a person can lose their moral compass completely.