In some cases, it could be a result of a mental breakdown. If Bella in the fanfiction has been through a great deal of stress or trauma, she might lose control and kill a human accidentally or without fully realizing the consequences. Also, there could be an evil influence, like an evil spirit possessing her, which forces her to commit the act against her will.
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.
Yes, it can be quite disturbing. Killing is an extreme act, and in fanfiction, it might go against the usual perception of a character like Bella. It could also involve themes of violence and moral conflict, which are not easy to digest.
One possible motive could be a complete breakdown of the peace they had worked so hard to build. Maybe an outside force manipulates Gaara's mind, causing him to turn on Naruto. Another could be extreme jealousy over Naruto's popularity or power.
One possible motive could be extreme jealousy. Maybe in this fanfiction, Hinata is insanely jealous of Sakura's relationship with Naruto or her skills, and that drives her to the unthinkable act of killing.
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.
A possible motive could be that Naruto discovers some dark secret about Minato that makes him feel betrayed. For example, if he finds out that Minato was involved in some unethical experiments that led to the suffering of many people. Or perhaps Naruto is tricked by a powerful villain who manipulates his emotions and convinces him that Minato is the root of all his problems.
One possible motive could be mind control. Maybe an enemy manages to take over Kakashi's mind and forces him to harm Naruto. Another could be a misunderstanding. For example, if Naruto was thought to be possessed by a powerful and dangerous entity that Kakashi believed could only be stopped by killing Naruto.
One possible motive could be if Sasuke goes too far in his pursuit of power and becomes a threat to the entire ninja world that even Naruto can't reason with. Maybe Sasuke starts to kill innocent people without hesitation, and Naruto, as the protector of the village and the one who believes in peace, sees no other option but to stop him by killing. Another motive could be some kind of dark magic or mind control that makes Sasuke an uncontrollable force of destruction, forcing Naruto to take drastic measures.
One possible motive could be that Sakura is brainwashed. Maybe an enemy uses some kind of mind - control jutsu on her and she becomes a puppet to kill Sasuke. Another could be extreme jealousy. If she felt that Sasuke was slipping away from her in some way, in a very dark and twisted version of the story, it could lead to her killing him.
One possible motive could be self - defense. Morgana had turned to the dark side and was constantly causing harm to Camelot and its people. Merlin, as the protector of Camelot, might have seen killing her as the only way to stop her reign of terror. For example, she was plotting against Arthur and trying to bring down the kingdom. Merlin's loyalty to Arthur and Camelot could have driven him to this extreme decision.