A common motive could be the pursuit of power. Vampire children may be tired of being under the control of the head vampire parent and see killing them as a way to gain absolute power within the vampire hierarchy.
One possible story could be about a power struggle. The vampire children might feel oppressed by the head vampire parent's strict rules and old - fashioned ways. So, they decide to kill the parent to gain freedom and take over the vampire clan. For example, if the head vampire parent restricts the children from feeding on certain humans, the children may rebel.
One possible motive could be pure dominance. Vampires are often depicted as powerful and controlling creatures. Raping a man could be a way for the vampire to assert its power over a human.
Jealousy could be one motive. Maybe Katherine is jealous of Elena's popularity and relationships, so she turns her into a vampire to make her life as difficult as hers. Another motive could be power. By turning Elena, Katherine might be trying to gain some sort of control over her or use her for her own plans.
One possible motive could be revenge. If Buffy has previously defeated him or his plans, he might want to turn her into a vampire as a form of ultimate revenge, to make her into the very thing she has been fighting against.
One possible motive could be protection. Maybe Damon senses a danger approaching the reader and biting them is his way of marking them as under his protection. In the vampire world, a bite can be a sign of claiming someone.
Well, it's a disturbing topic. Sometimes, the caregivers might have untreated psychological disorders that lead them to such heinous acts. Or, maybe they are in a desperate financial situation and see it as a way out. Another factor could be a lack of proper support and resources, causing them to snap.
Maybe Naruto forms an alliance with some of the main characters in Rosario Vampire. He could use his ninja skills combined with his vampire abilities to fight against common enemies. Like, if there's a powerful dark force threatening both the vampire and human worlds, Naruto could be a key player in the defense.
A possible motive could be revenge. If a dragon had hurt someone he loved, like his father or Astrid, he might be driven to kill it. It would be a very emotional and difficult decision for him, but his love for his friends and family could push him to that extreme.
One possible motive could be jealousy. Hermione is highly intelligent and often outshines Draco in academics. In a twisted version of a rivalry, Draco's envy could turn into a deadly hatred. Another could be his family's influence. If the Malfoys push him to take extreme actions against Muggle - borns, Hermione would be a prime target.
One possible motive could be jealousy. If Kiba were to gain more recognition in the village than Naruto in the fanfiction world, Naruto's insecurities could drive him to do something drastic like killing Kiba. But this is a huge departure from his usual self.