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.
Perhaps it's a sacrifice play. In the fanfiction, the Uchia compound could be under attack by a powerful enemy. Sasuke might be willing to sacrifice himself to save the compound, but Naruto misunderstands his intentions. Naruto, thinking Sasuke is making a wrong and dangerous move, tries to stop him and ends up killing him instead.
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.
Yes, such fanfictions exist. In these fictional scenarios, Naruto might kill himself because of Sasuke for several reasons. Naruto's whole life has been centered around Sasuke to some extent. If he feels that he has failed in his mission to bring Sasuke back to the right path and that the situation has become so dire that the only way to atone for his 'failure' is through his death, he might do it. Also, if Sasuke were to cause harm to someone Naruto holds dear, like the village or his other friends, and Naruto feels powerless to stop him in any other way, he could choose to end his own life as a last resort.
One possible reason could be that Sasuke might have gone too far in his pursuit of power even after the war. He could have been a threat to the newfound peace in the ninja world, and Naruto, as the protector of peace, might have felt he had no other choice.
One possible reason could be a sudden and extreme corruption of Naruto's character. Maybe he is influenced by some dark and powerful force that makes him turn against his friends. Another reason might be a misinterpretation of a prophecy or some kind of mind - control situation.
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.
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.
One possible motive could be if Sasuke goes too far in his pursuit of power and becomes a threat to the entire ninja world that even Naruto can't reason with. Maybe Sasuke starts to kill innocent people without hesitation, and Naruto, as the protector of the village and the one who believes in peace, sees no other option but to stop him by killing. Another motive could be some kind of dark magic or mind control that makes Sasuke an uncontrollable force of destruction, forcing Naruto to take drastic measures.
It could also be due to a misunderstanding. In the heat of the moment during the vote, there might have been miscommunications or false assumptions. Naruto might have thought Sasuke was going to betray them all and in his haste to prevent that, made the tragic decision to kill him.
Well, in a really dark and twisted fanfiction, Sasuke could be under some sort of mind - control spell. The spell could be so strong that it overrides his feelings for Naruto. He might then attack and kill Naruto without fully realizing what he's doing. But in the real Naruto lore, their friendship and mutual respect are so strong that this is highly unlikely.
Another reason might be related to their different ideologies within the ANBU. Naruto may have believed in a more collaborative and friendly approach within the organization, while Sasuke might have been more focused on individual achievement and power. This fundamental difference could lead to Naruto hating Sasuke in the fanfiction.