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 miscommunication that escalates out of control. Maybe Sasuke is on a path that Naruto believes is too dangerous for the world, and Naruto, in an attempt to stop him, accidentally kills him. For example, Sasuke might be about to use a powerful and uncontrollable jutsu that Naruto fears will destroy the Uchia compound and much more, so he acts in haste.
Kakashi would be deeply affected. He has mentored both Sasuke and Sakura. This would make him question his own teachings and his ability to protect his students. He might also feel a sense of responsibility and could become more involved in trying to bring Sasuke to justice or at least understand what drove him to such an extreme act.
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.
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.
For Kakashi, it would be like losing his student all over again. He has trained Naruto and seen his growth. His death would make Kakashi question his own role as a teacher and mentor. He might become more withdrawn and blame himself for not being able to prevent Naruto's suicide.
It would surely cause a great shock among the different factions in the Naruto universe. The existence of an angel attendant might have been something that many held in high regard or were even afraid of. Naruto killing one would make him a controversial figure. Some might see him as a hero for eliminating a potential threat, while others might view him as a murderer.
The other students would likely spread rumors. Some might idolize Hermione for her bravery in taking on a troll, while others might be afraid of her. This could lead to a change in her social standing at Hogwarts. She may become more isolated as some students avoid her, or she could gain a group of followers who admire her boldness.
It would likely shock the fan community. Since Naruto and Sasuke are such iconic characters, any major deviation like this would cause a stir. Some fans might be outraged as they are attached to the characters' relationship. Others might be intrigued by the new and dark take on the story.
In a fanfiction where Naruto kills Sasuke at the Uchiha compound, it's a very dark turn. Naruto's character would be severely damaged in the eyes of many. His journey was about saving Sasuke, not killing him. This could also lead to a power vacuum as Sasuke was a powerful shinobi. The Uchiha clan's already tragic story would become even more so. It might also cause Naruto to lose his way, as Sasuke was a sort of mirror to him, and without Sasuke, he might lose his motivation to keep getting stronger.
Harry would be deeply affected. Snape was a complex figure in Harry's life, and Remus was also a mentor. Harry would be torn between his loyalty to Remus and his feelings for Snape. He might start to question everything he knew about both of them. Hermione and Ron would also be shocked. They would likely try to understand Remus' reasons but also struggle with the fact that he committed such a heinous act.