Another motive could be self - preservation. If Raven saw the OC as a threat to her own goals or existence, she might have attacked first. In self - defense, the OC could end up killing Raven. Also, the OC might be part of a group that Raven was targeting, and killing her was the only way to ensure their safety.
The relationships of the other characters would be greatly affected. Yang, for instance, would be devastated as Raven was her mother. This could lead to a complex dynamic between Yang and the OC. Yang might initially be full of hatred towards the OC, but as the story progresses, she could come to understand the OC's reasons.
One possible motive could be misinformation. Maybe Bucky was misled to believe the OC was a threat to someone he cares about. So, he kidnapped the OC to prevent them from causing harm.
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.
Well, in this kind of fanfiction, Doc Oc could kidnap Spiderman to force him to reveal his identity. Doc Oc might think that knowing who Spiderman really is would give him an upper hand in their ongoing battles. It could also be that Doc Oc wants to study Spiderman's powers. He's a scientist after all, and Spiderman's abilities are quite unique. He might believe that by kidnapping and examining Spiderman, he can find a way to replicate or counter those powers.
It could be that Raven is being manipulated by someone else. In the RWBY universe, there are many forces at play. Another character could be blackmailing her or using some form of mind control to make her cheat on the reader. This would add an extra layer of complexity to the story and make Raven not entirely at fault, at least initially.
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.
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.