Maybe there's a misunderstanding. For instance, if Bella is involved in some secret or dangerous situation that Mike misinterprets as something harmful to the people he cares about, he could attack her. Another reason could be that Mike is being influenced by some external force in the fanfiction world, like a dark magic or an evil entity that makes him act out of character and attack Bella.
In some fanfictions, the motive for Mike to attack Bella could be related to power struggles. If there's a hierarchy or some sort of power dynamic in the story's setting, and Bella represents a threat to Mike's position or the status quo, he might try to attack her to eliminate that threat. Also, it could be a case of misinformation spread about Bella, which makes Mike believe she is a danger and thus attacks her.
One possible motive could be jealousy. If Mike has feelings for Bella and she is interested in someone else, like Edward in the Twilight series, he might be driven by envy to attack her.
Perhaps in the fanfiction, Bella has been influenced by a darker force. It could be a new vampire or some other supernatural entity that has turned her into a bully. This external influence could be warping her mind and making her act out against those she once cared about or new people in Forks.
One possible motive is to rescue someone. Maybe a friend or a family member of theirs has been captured by the Volturi. Another could be to gain knowledge. The Volturi are very old and have a lot of secrets about vampire existence, so they might go there to learn more.
It could be revenge. If the human had wronged Bella in some way, like harming her family or friends in the fanfiction world, she might be driven to kill out of a sense of vengeance. Or perhaps she is forced into a situation where killing seems like the only option to protect a greater good, like saving a whole community from a dangerous human.
It could be that Mikael has some sort of power or knowledge that Bella doesn't want him to use. If he's planning something evil or dangerous in the context of the fanfiction, Bella might decide to eliminate him to prevent chaos or harm. For example, if he's about to release a powerful dark force that would destroy both her world and the world he comes from. Another possibility is that Bella has been manipulated into believing that Mikael is a greater evil than he actually is, perhaps by an antagonist who wants Mikael out of the way for their own reasons.
Power could be a motive. She might want to have control over the vampire world or the humans in Forks. So she starts doing villainous things to gain that power.
Well, in some fanfictions, Harry might attack Dumbledore if he discovers that Dumbledore has been using him as a pawn in a bigger game all along. This feeling of betrayal could drive him to lash out. Also, if there is a prophecy that he misinterprets to mean Dumbledore is a threat to his future or the future of his friends, that could be a motive.
One common theme is unrequited love. Since in the original story Mike has feelings for Bella that she doesn't fully return, many fanfictions explore this aspect further. Another theme is friendship. Some stories focus on how they could have been really good friends if not for the romantic complications.
Good bella and mike lemon fanfiction often has well - developed characters. For example, they might give Mike more depth than in the original, showing his real emotions towards Bella.
One possible plot could be that Mike makes an inappropriate comment about Bella's relationship with someone else, and in a fit of anger, she punches him. Then the story could follow the consequences of that action, like how their friends react or if there are any apologies later.
Well, some could be about their growth as individuals. Maybe Mike is trying to overcome his shyness and Bella is helping him. Or it could be the other way around, with Bella learning a new skill or hobby with Mike's support. There could also be fanfictions that are set in a fantasy world, like a magical realm where they have special powers and they use those powers to protect their world from an evil force.