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.
Maybe in the fanfiction, Harry discovers some dark secrets about his parents. For example, they could have been involved in something unethical before they died. This new knowledge could make him feel disgusted and lead to hatred. He might also feel pressured by the heroic image of his parents that everyone around him seems to uphold, and when he can't live up to that, he turns that frustration into hatred for them.
In the fanfiction world, it could be that Harry has been influenced by some dark magic or a malevolent character. This external influence makes him see his parents in a negative light. He might be made to believe false things about them, like they were selfish and only cared about their own glory in the wizarding war, rather than their family.
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.
One possible reason could be the betrayal he faced from some people in the wizarding world. For example, some students at Hogwarts might have spread false rumors about him or treated him unfairly because of his fame as 'The Boy Who Lived'. This could have led him to develop a general dislike for humans.
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.
One possible reason could be the way the Dursleys treated him. They were cruel, made him live in a cupboard, and constantly belittled him. This kind of mistreatment over the years could easily lead to hatred. Another aspect could be the lack of connection to his wizarding family that he was not aware of for a long time. He might feel angry at his parents for leaving him in such a situation without any real protection from the Dursleys.
One reason could be that he doesn't like the spotlight that comes with being a Gryffindor. In the original story, Gryffindors are often in the limelight, and in fanfiction, he might be tired of all the attention.
In this fanfiction, perhaps there's some miscommunication. Maybe the Weasleys' boisterous nature is misconstrued by Harry. For example, their large family dynamic might be overwhelming to him at times, which could be a reason for the 'hate' in the fanfic.
One possible reason could be that in the fanfiction, her parents might have made a sacrifice that left her alone, and she resents them for not being there for her. Maybe she feels abandoned and that has led to her hatred.
She might hate it because it could change the way people see her character. In the original story, she is a strong and independent person, but fanfiction might portray her in a wrong way.
One possible reason could be a miscommunication. Maybe Hermione accidentally reveals a secret of Harry's that he didn't want known, and this causes him to feel betrayed and start to hate her. Another reason could be jealousy. If Hermione gets more attention or praise for her magical abilities than Harry in the fanfic, he might develop feelings of envy and hatred. Or perhaps there's a love triangle situation where Hermione is involved with someone Harry doesn't like, and that causes his hatred.
It could also be that the fanfic might interfere with the way he wants his story to be remembered. His adventures at Hogwarts were real and important to him, and when fanfic changes the narrative, it's like his story is being disrespected. For example, if a fanfic portrays him as cowardly when he was actually brave, that would surely make him dislike it.