Jealousy. Maybe the brothers were jealous of the attention the girl gave to her lover. They felt left out or that the lover was taking their sister away from them. So, in a fit of envy, they killed the lover.
Family honor. In Spanish culture, family honor is highly valued. If the lover was from a different social class or a family with a bad reputation, the brothers might kill him to protect the family's honor.
In a Spanish story, perhaps the girl's family was of high social standing. Her lover was from a lower - class family. Her brothers, who were concerned about the family's reputation, decided to get rid of the lover. They lured him to a secluded place under the pretense of a friendly meeting. Then, without mercy, they killed him. The girl, upon finding out, was devastated and spent the rest of her life mourning her lost love.
It could be a revenge plot gone too far. Suppose her parents did something in the past that she just found out about, and in her new dark persona, she decides to take the ultimate revenge. Or perhaps she has fallen so deep into the dark side that she no longer values family and sees them as just another obstacle in her path of darkness.
One possible motive could be ransom. The kidnappers may think the girl's family is wealthy and they can get a large sum of money in exchange for her safe return.
Mental illness could be a motive. For example, schizophrenia might make her have hallucinations or delusions that her parents were a threat.
One possible motive could be envy. If the cheerleader was popular, successful, or had something that others desired, envy might have driven someone to have such a dark thought.
Revenge can play a role too. If one friend feels wronged by the other, maybe through an act of betrayal or a hurtful comment in the past, they could seek revenge in the most extreme way, which is murder.
One possible motive could be a desire for friendship. Maybe one girl sees qualities in the other that she admires and wants to be close to her, like in the case where one girl is attracted to the other's creativity and tries to 'seduce' her into a friendship by sharing related interests.
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.