Perhaps in the fanfic, Brick has some sort of relationship with Blossom that she doesn't approve of. It could be a forced romantic relationship or a partnership where Brick is constantly overshadowing Blossom or not respecting her. Another aspect could be that the fanfic changes the fundamental nature of the world they exist in in a way that Blossom finds unacceptable, and Brick is somehow a symbol of that change.
One way could be for the writer of the fanfic to rewrite Brick's character to be more palatable to Blossom. Maybe make him more heroic or less antagonistic.
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.
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.
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.
One reason could be that some fans want to explore different character dynamics. The original Bella - Edward relationship is so well - known and often idealized, so making Bella hate Edward creates a whole new set of tensions and story possibilities.
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.
Maybe Ash has some misunderstandings about Dawn. For example, he might think Dawn is too competitive and it makes him uncomfortable.
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 the fanfic, Naruto could hate the Hokage because of some bad experiences related to past Hokages. Perhaps he discovers some dark secrets about how the Hokage office has made decisions that he doesn't agree with. It might be something like a cover - up of certain events that Naruto believes should have been dealt with more fairly.
One reason could be creative exploration. Writers might want to see how the story would unfold if Naruto hated Hinata. It's a way to break away from the normal storyline.