For Kakashi, it would be a nightmare come true. He has trained both Naruto and Sasuke and has always hoped they could find a way to coexist and even become friends again. If Naruto almost kills Sasuke, Kakashi would feel like he has failed as a mentor. He would likely try to step in and fix the situation, using his wisdom and experience to try and make Naruto see reason.
The rest of the Konoha villagers would be in shock. Naruto is seen as a hero in the village, and Sasuke was once a part of the village too. The idea of Naruto turning on Sasuke like that would make the villagers question Naruto's judgment and stability. It could also create a divide among those who support Naruto's actions and those who think he's gone too far.
Sakura would be devastated. She has always had deep feelings for Sasuke. She might initially be angry with Naruto, thinking that there could have been another way. But later, she might understand Naruto's difficult decision when she realizes the threat Sasuke posed.
For characters like Itachi, if his spirit or some form of his consciousness was still around in the fanfiction, it would be a complex situation. He sacrificed so much to protect Sasuke, and now Naruto killing him would be a huge irony. Itachi might try to influence the events from the afterlife or through some other means. And Hinata, who loves Naruto, would be deeply concerned for Naruto's state of mind. She would try to support him, but also struggle with the moral implications of what he has done.
One possible reason could be a misunderstanding. Maybe Sasuke is framed for something really bad, and Naruto, in his anger and sense of justice, lashes out without knowing the truth. For example, if there's a false rumor that Sasuke has attacked the village again, Naruto might be quick to act.
The impact on Hinata would also be significant. She understands Naruto's sense of justice but would also be worried about the emotional toll it would take on him. She might try to comfort Naruto while also being conflicted about the whole situation. For the other ninja clans in Konoha, it could lead to internal disputes. Some clans might see Naruto's action as necessary for the safety of the village, while others might view it as a breach of the bond between comrades. And for the ninja world at large, it could change the power balance. Sasuke was a very powerful ninja, and his death at Naruto's hands could make other villages either fear Konoha more or see it as a place where even the closest friends can turn on each other.
The demigods would be in shock. Percy was a leader among them. His death at the hands of Artemis would make them question the motives and stability of the gods. Annabeth, in particular, would be devastated. She would likely be consumed by grief and then anger, and might even set out on a quest for revenge against Artemis.
As for the other Konoha 11 members, they might initially be skeptical. But as they see how Sasuke is changing for the better under Naruto's adoption, they start to include him more in group activities. They come to respect Naruto's decision and see Sasuke in a new light, which gradually strengthens the overall unity among them.
The village of Konoha would be in chaos. Naruto was seen as a hero, and if his dark side kills Sasuke, it would shatter the trust of many. His friends like Sakura would be devastated. She might go on a revenge quest against Dark Naruto. The Uchiha clan, which Sasuke was part of, would also be in turmoil. They might see this as an act of aggression against their clan and seek justice.
Their relationship would likely become very strained. Kiba would probably be afraid of Naruto at first. He might avoid Naruto and have a hard time trusting him again. Naruto, on the other hand, would feel extremely guilty.
For Sakura, it could be a huge emotional blow. She might feel abandoned and be motivated to become stronger herself to bring them back. Also, she might have to step up in their absence and take on more responsibilities in the village.
It could make their peers jealous. For example, characters like Shikamaru might feel that Naruto and Sasuke are getting special treatment. But for their mentors, it could strengthen the bond. Kakashi might have more faith in them and train them more intensively. And for Sakura, she might be both proud and a bit envious of Sasuke's early graduation.