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, 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.
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.
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.
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.
One possible event could be that Kurenai accidentally endangers one of Naruto's loved ones during a mission. This would make Naruto angry at her.
In fanfiction, a scenario could be that the Cullens exclude Bella from important family decisions. This makes her feel like an outsider and unloved. Or perhaps they try to manipulate her into doing something she doesn't want to do, such as leaving her human friends behind. This would lead to her not forgiving them.
I'm not sure specifically as there could be many different takes on it. Generally, it might be about Bella using her singing in a context related to the Cullen family in a fan - created story.
It's possible that in the fanfiction, Zeus made a deal that put Percy or his loved ones in danger. For instance, Zeus might have made an agreement with an enemy just to maintain his own power, without considering the consequences for Percy and those around him. This would surely make Percy hate Zeus.
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.