Self - defense is also common. If the guy is attacking the girl, she may have no choice but to fight back and end up killing him to protect herself. For example, in cases of domestic abuse, the girl may finally snap after years of being mistreated.
One such story could be in a fictional thriller setting. There was a girl named Lily. She was framed by a guy named Tom for a crime she didn't commit. In order to clear her name and get revenge, she carefully planned. She lured Tom to an abandoned building. When he arrived, she confronted him with evidence of his misdeeds. Tom, in a fit of rage, attacked her, but she was prepared and managed to use his own weapon against him, ultimately killing him in self - defense.
One possible motive could be financial security. Older guys are often more established in their careers and might have more resources. For example, the girl might see the older guy as a way to live a more comfortable life.
A possible motive could be a creative exploration of the 'what - if' scenarios. Maybe the writer wants to see how the story would unfold if Sasuke completely lost his way. Another motive could be to create a sense of high - stakes drama. Since Sakura is an important character, her death at Sasuke's hands would be a huge shock.
One possible motive could be power. In some fanfics, Sasuke might be driven by an insatiable thirst for power and see Naruto as an obstacle. Another could be revenge, perhaps believing Naruto has wronged him in some way in that alternate story. Or it could be a matter of being corrupted by dark forces, which make him turn against Naruto.
Love or obsession could be a motive. Just like in some stories where a girl is so in love with a boy that she kidnaps him thinking it's a way to keep him close.
Perhaps the manga explores their daily lives, the guy's internal conflicts, and how the girl influences his decisions. It might also feature supernatural elements related to the tsukimonogami that continue to haunt them.
One possible motive could be a complete breakdown of the peace they had worked so hard to build. Maybe an outside force manipulates Gaara's mind, causing him to turn on Naruto. Another could be extreme jealousy over Naruto's popularity or power.
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.
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.
One possible motive could be to explore the darker side of the Tails character. Maybe the writer wants to show that even a seemingly positive and energetic character like Tails can have internal struggles.