Well, it could be due to the way the Cullens interact with the other people in Forks. If Bella notices that they are being manipulative or using their powers in a way that she doesn't like to influence the townspeople, that could make her start to hate them. Also, if there is some kind of power struggle within the Cullen family that starts to affect Bella negatively, like she gets caught in the middle of it or she sees how it's causing harm to those she cares about, she might turn against them.
One possible reason is that the fanfiction distorts the original relationship between Bella and Sam in a way she finds unacceptable. For instance, if it creates a false romantic tension where there shouldn't be one.
In some fanfictions, Bella might accidentally harm one of the Cullens, either physically or emotionally. This could lead to them hating her. Also, if she were to side with their enemies in a conflict, that would definitely make them dislike her. For example, if there was a feud between two vampire groups and Bella chose the wrong side, the Cullens would not be happy with her.
It's possible that in the fanfiction, the Cullens are involved in some sort of betrayal. For example, they might side with someone who is against Bella. This act of betrayal would be a strong reason for her to hate them. In fanfiction, new storylines can be created where the Cullens' actions are not in line with what Bella expects or desires, which can result in her hatred.
In some fanfictions, the Cullens might hate Bella because of jealousy. Maybe other vampires are jealous of the attention she gets from Edward. Another reason could be that she brings too much danger to their family. Her presence could attract unwanted attention from other vampires or vampire hunters, which puts the Cullens at risk.
In the original Twilight story, Bella is initially attracted to the Cullens, especially Edward. But in this fanfiction where she starts to hate them, it's a complete contrast. It shows a different side of Bella's character, one that is more critical and less forgiving. In the original, she is more likely to overlook their flaws because of her love for Edward. Here, she focuses on the negative aspects.
One reason could be a desire for a more edgy or non - traditional take on the popular Edward and Bella relationship. Some people might want to explore a darker side of their dynamic that goes against the typical romantic narrative.
One possible reason could be that she discovers some dark secrets about the Cullens. Maybe she finds out they have been lying to her about their true nature in a more malicious way than in the original story. Another reason might be that she feels threatened by their vampire powers. If they accidentally or purposefully use their powers around her in a menacing way, it could make her feel scared and turn that fear into hatred.
One consequence could be a broken family relationship. Their interactions would be filled with tension and animosity. Family gatherings would become uncomfortable, and other family members might be forced to take sides.
Bella's hatred could also lead to her making rash decisions. For example, she might get involved with the wrong crowd or engage in self - destructive behavior just to spite her mother. And this could further complicate the story in the fanfiction.
In this fanfiction, it might be that Bella has received some sort of premonition. She could have seen a future where staying with the Cullens leads to their destruction. Out of love for them, she chooses to say goodbye. She might plan to go far away, change her identity, and live a life away from the supernatural world that has been both alluring and dangerous for her.