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.
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 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.
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.
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.
In a 'Harry Potter kills Lucius Malfoy' fanfiction, the motives could be complex. It could start with Lucius's long - standing enmity towards Harry and his family. Harry may have reached a breaking point after all the years of Lucius's attempts to undermine him. Additionally, in the context of the wizarding war, if Lucius is seen as a key figure in Voldemort's regime, Harry might view his death as a strategic move to weaken the Dark Lord's forces. It could also be a way for Harry to assert his own power and dominance over the Malfoy family, who have always looked down on him.
One possible motive could be revenge. Bellatrix killed Sirius, who was very dear to Harry. So in the fanfiction, Harry might be driven by the intense desire to avenge Sirius' death.
One possible motive could be revenge. If Pauna had done something really bad to Clarke or someone Clarke cares about in the fanfic's backstory, Clarke might feel compelled to kill Pauna. Another could be self - defense. Maybe Pauna was attacking Clarke and she had no other choice.