Another reason could be that she has been influenced by some dark force. In the Naruto universe, there are many evil entities that can manipulate people's minds. If she falls under the influence of such a force, she might end up hurting Naruto without really meaning to in her normal state of mind.
Maybe she's jealous of Naruto's popularity or his strength. It's possible that she feels overshadowed by him all the time and that jealousy turns into resentment, leading her to hurt Naruto.
Another way is if his sister has some special powers that she can't fully control yet in the fanfiction world. And during a training session or an accidental encounter, her powers lash out and end up hurting Naruto.
One reason could be jealousy. If Naruto is getting more attention or has achieved more in the ninja world, his sister might be jealous and this could lead to her actions that hurt Naruto, either by spreading false rumors or directly confronting him in a hurtful way.
Well, Naruto could also seek support from his friends like Sasuke or Sakura. They have been through a lot together, and they might be able to offer him some comfort and advice on how to handle this difficult situation. Since Naruto values his friendships highly, their words could have a big impact on him.
In some fanfictions, Percy's sister might hurt him emotionally by betraying his trust. For example, she could share his secrets with others, making him feel exposed and betrayed.
She could be an original character created specifically for this fanfiction. The author might give her unique abilities and a distinct personality to make the story more interesting.
One possible reason could be to protect her. Naruto might believe that by leaving, he can draw danger away from his sister. For example, if there are powerful enemies targeting him, he doesn't want his sister to get caught in the crossfire.
In the Naruto series, Naruto doesn't have a biological sister in the original story. So in this fanfiction, the author probably creates a new character to be his sister. It could be an original character with unique abilities and a complex backstory that leads to the situation where Naruto dies by her hand.
A common scenario could be over inheritance or family heirlooms. If there's something valuable in the family that both Percy and his sister want, she might scheme to get it all for herself, leaving Percy feeling cheated and hurt.
One possible reason could be a misunderstanding. Maybe Naruto was misidentified as a threat in a tense situation. For example, if there was some sort of dark jutsu at play that clouded Kurenai's judgment and made Naruto seem like an enemy. Another reason might be that Kurenai was under some sort of mind - control. There are many villains in the Naruto universe who are capable of such things. If she was being controlled, she might unknowingly attack Naruto.
In this fanfiction, perhaps Naruto's sister has some special power. Naruto loves her not only because she's family but also because he sees the good in her power. He might teach her how to control it and use it for good. They could go on adventures together, with Naruto always being the over - protective brother and making sure she doesn't get into too much trouble.