It could be revenge. Maybe one character was wronged and sought justice through killing.
Sometimes, it's to show the dark side of a fictional world. It adds drama and complexity. Or it could be a result of a character's mental instability.
It could be a form of exploring darker themes. The writer might be interested in how the RWBY world would react if Ruby Rose was killed. They might want to see how other characters would develop or change in the face of such a tragedy. However, this is a rather extreme way of exploring those themes and might not be well - received by most fans.
Maybe the husband was mentally unstable and had a sudden burst of violence. Or, there could be a deep-seated jealousy issue related to something the wife did.
Manson's motives were deeply rooted in his cult - like thinking. He was obsessed with creating chaos and upheaval. The pregnant woman, Sharon Tate, was a high - profile target. Manson saw her as a way to send a message to the society he despised. He thought that by having her killed, along with others, it would be a catalyst for the race war he envisioned. His followers blindly followed his orders, believing in his warped ideas. This all shows how dangerous his motives were and how his cult - like influence led to such a tragic and brutal murder.
One possible motive could be mind control. Maybe an enemy used a powerful genjutsu on Naruto, forcing him to commit this heinous act against the Uchiha clan.
Maybe he wanted to leave some kind of evidence or confession. Or perhaps he was mentally unstable and saw it as a twisted way to make his death known.
The main motive could be to regain control and stop the trauma. But it could lead to her being judged by society, or having to live with the guilt. Maybe it also prompts changes in the law regarding victims' actions.
One possible motive could be his thirst for power. By kidnapping someone with a special ability or knowledge, like a powerful Force - user or a brilliant scientist, he could gain an edge in his quest to rule the galaxy. For example, if he kidnapped a Jedi who knew the location of ancient Sith artifacts, he could use those artifacts to increase his own strength.
One possible motive could be a conflict over their loyalties. Remus is a werewolf and has his own set of insecurities and allegiances. Maybe Snape discovers something about Remus that he threatens to expose, leading Remus to kill him in a moment of panic. For example, if Snape were to find out about Remus endangering students during his werewolf transformations and was about to report him to the authorities.
One possible motive could be his fear of losing her. In the Star Wars universe, Anakin is deeply in love with Padme. If in fanfiction he feels that she is in danger or might be taken away from him by some external force or person, his desperation could lead him to kidnap her in an attempt to keep her safe. For example, if he suspects the Jedi Council of having some secret plan regarding Padme, his unstable state of mind might push him to this extreme action.
Well, Ursula could kidnap Ariel in fanfiction because she wants to take Ariel's place. Think about it. Ariel has a loving family, a great voice, and is adored by many. Ursula, on the other hand, is often shunned. So she might think that by kidnapping Ariel and getting rid of her, she can step into Ariel's life. Maybe she wants to be the one who is loved by Prince Eric. Or perhaps she wants to be the favorite daughter of King Triton. It's all about her twisted desires for a different kind of life.