There could be several reasons. Perhaps Paul doesn't like the writing style of most Bella fanfiction. It might be too sappy or melodramatic for his taste. Or maybe he has some personal issues with the concept of fanfiction in general, and since Bella is a popular character, her fanfiction is just an example of what he dislikes about the whole fanfiction genre.
Well, it could be that Paul has a different vision of the story's original plot and the fanfiction of Bella goes against that. He might be a purist when it comes to the original story and detest any deviation in the form of fan - made fiction about Bella.
Well, in some fanfictions, Hermione might hate Bella because their personalities are so different. Hermione is studious, logical, and a fighter for justice. Bella, on the other hand, is often portrayed as more passive and overly focused on her romantic relationships. Their different values and ways of approaching life could lead to Hermione's dislike of her in fanfiction.
In some fanfictions, Jacob might hate Bella because of the love triangle situation. He could feel that Bella has led him on or toyed with his emotions. For example, if she constantly goes back and forth between him and Edward, it can make Jacob bitter and resentful towards her.
In some fanfiction, Bella might hate Jacob because of his over - protectiveness. He can be very possessive and that could make Bella feel suffocated, leading to dislike.
Maybe Elena just doesn't like the way Bella is portrayed in fanfiction. It could be that she has a different view of the original Bella character and the fan - made versions deviate too much from it.
Maybe they have a personal dislike for the character Percy. It could be that they find the fanfiction versions of Percy to be too far from their idea of what the original character should be like.
Another reason could be the influence of other characters. There might be a new character in the fanfiction who convinces Bella that Edward is not who she thought he was. This new character could plant seeds of doubt in Bella's mind about Edward's intentions and actions, ultimately resulting in her hatred.
It could be that in the fanfiction world, Bella's mother has made some bad decisions in the past that directly affected Bella negatively. Perhaps she abandoned Bella at a crucial time, which led to Bella's hatred.
It could be because of a misunderstanding. Perhaps there was a miscommunication or an incident where the sister was wrongly blamed for something, and Bella held onto that grudge. Say the sister accidentally broke Bella's favorite item but Bella thought it was done on purpose, which then festered into hatred in the fanfiction world.
Maybe they have a different taste in literature. They might prefer other types of stories and not be into the fanfiction genre, especially those related to Percy and Annabeth.
Well, it could be that in the fanfic's storyline, Bella has had some bad experiences in the past related to intimacy. Maybe she was betrayed or hurt, which made her develop this hatred.