Well, it might be because of jealousy. Suppose there's another female character in the fanfiction who has a certain allure. Inuyasha could be momentarily swayed by her charm and in the process, betray Kagome. Also, a prophecy or a so - called 'fate' in the fanfiction world might drive Inuyasha to betray Kagome. Like there's a prediction that says his association with Kagome will lead to some great disaster, so he decides to distance himself in a very cruel way which seems like betrayal.
One possible reason could be a misunderstanding. Maybe Inuyasha misinterprets Kagome's actions or words. For example, if Kagome is trying to help a new character who seems to have a connection to the Shikon Jewel, Inuyasha might think she's being disloyal and thus betray her. Another reason could be some sort of dark influence. There could be an evil force that makes Inuyasha act out of character and turn against Kagome.
In some fanfictions, it could be due to Inuyasha's own insecurities. He might feel that he is not worthy of Kagome, perhaps because of his half - demon status. And instead of dealing with these feelings in a healthy way, he pushes Kagome away by betraying her. It could also be a plot device used by the author to create drama. By having Inuyasha betray Kagome, the author can introduce a lot of conflict, like Kagome's reaction, how she tries to get revenge or get him back, and how it affects the relationships with other characters in the story.
There could be several reasons for the existence of such fanfiction. Firstly, it might be a form of creative exploration for those who want to test the boundaries of the characters' personalities. They may think that by putting Inuyasha in an 'abusive' role, they can dig deeper into his character flaws. Secondly, it could be an attempt to create a contrast. Since the original relationship between Inuyasha and Kagome is mostly positive, showing an abusive side can create a sharp contrast. However, this kind of fanfiction often goes against the general perception and liking of the majority of fans.
One possible reason could be a misunderstanding. Maybe Kikyo was in some kind of danger and Inuyasha's actions to save her were misconstrued as betrayal. Another reason might be that Inuyasha was influenced by some external dark magic that clouded his mind and made him act against his normal feelings for Kagome.
Perhaps Sesshomaru sees some value in Kagome. Maybe he realizes that she has certain powers or knowledge that could be useful to him in the future, so he decides to save her from Inuyasha.
Kagome could also be pressured by outside forces in the fanfiction world. Maybe her family or friends in her own time have certain expectations of her that conflict with her relationship with Inuyasha. So, to please them or to avoid getting in trouble, she might do things that hurt Inuyasha, like breaking off their relationship suddenly or treating him coldly.
Perhaps there is some sort of political intrigue within the 'light side'. There could be factions that see Hermione as a threat to their own power and influence. So, in order to maintain their position, they choose to betray her.
One possible reason could be jealousy. Maybe her friends are envious of Hermione's intelligence and magical abilities. In a fanfiction, this could lead them to betray her to gain an advantage or to make themselves look better. Another reason might be a misunderstanding. A miscommunication or false assumption could cause her friends to turn against her.
Kagome might respond with anger. She could be furious at Inuyasha for his betrayal and might lash out at him. She might use her spiritual powers against him in a fit of rage.
It could also be due to competition. Sabrina might see Ash as a threat to her status as a powerful trainer. If Ash starts to gain more recognition and success, Sabrina, being prideful, could feel betrayed as if Ash is trying to overshadow her and thus take actions against him.
One reason could be power. Maybe he was offered a great deal of power by an outside force in exchange for betraying the harem. Another could be jealousy. If there was someone in the harem who was getting more attention or was more successful than him in some way, he might have been driven by envy to betray them.
Well, in fanfiction, the author often likes to create complex and unexpected plot twists. It could be that Naruto's loyalty to his own values and beliefs clashes with the goals and ethics of Rias, Sona, and Ravel. For instance, Naruto may be against certain practices that they engage in, like how they handle power or deal with other groups. This ideological difference could lead to their betrayal. Also, there could be a love triangle or some sort of relationship drama involved. If Naruto is involved with someone they all like or if there are misunderstandings in relationships, it could spark the betrayal.