There could be some new, unnamed vampire characters introduced in fanfiction who are part of a rival vampire group. They might see Moka as a threat to their group's plans or simply as a target to gain leverage over the other vampires at the school. Their motives would be based on the politics and power struggles within the vampire underworld in the fan - created story.
Kokoa might also be considered. She has a complex relationship with Moka. If she felt that Moka was getting too much attention or if she believed that Moka was in her way of achieving something, like gaining more power within the family or among the vampires at school, she could betray her sister. Kokoa's impulsive nature could drive her to make such a hasty decision.