Sasuke would be extremely angry. He and Naruto had a complex relationship that evolved from rivalry to friendship. His first reaction would likely be to seek vengeance. Kakashi himself would be filled with guilt and self - loathing. He has always been Naruto's teacher and protector in a sense. Hinata would be heartbroken. She had deep feelings for Naruto. The rest of the village would be in shock and disbelief. Naruto was seen as their future, and his sudden death at Kakashi's hands would create a sense of despair and confusion.
One reason could be a misunderstanding. Maybe some false information makes Naruto think Kakashi is a traitor to Konoha. So, in a fit of rage, he kills Kakashi. Another reason could be related to power. If Naruto becomes obsessed with power and Kakashi tries to stop him from going down a dark path, Naruto might lash out and kill him.
Perhaps there is a situation where Sakura is trying to protect Naruto from a dangerous attack, but due to some unforeseen circumstances, like a misdirected healing jutsu turning into a harmful one, she ends up killing him. It could then lead to a story of guilt, revenge, and the search for a way to bring Naruto back.
Well, if there is a 'naruto kills kakashi fanfiction', it would be a very different take on the Naruto universe. It could be set in a world where Naruto has been corrupted by an evil force. This force could be manipulating Naruto's emotions and memories, making him think Kakashi is a threat. In the original series, they have a strong bond, so this would be a very controversial and unexpected plot.
One possible motive could be mind control. Maybe an enemy manages to take over Kakashi's mind and forces him to harm Naruto. Another could be a misunderstanding. For example, if Naruto was thought to be possessed by a powerful and dangerous entity that Kakashi believed could only be stopped by killing Naruto.
Well, it would have a huge impact. Firstly, it would shock the other characters. Naruto is a central figure, and her death would create a power vacuum. Sasuke would likely be consumed by guilt, which could lead to him either isolating himself or trying to make amends in extreme ways. It could also change the direction of the whole fanfiction world. For example, the village might descend into chaos without Naruto's unifying presence.
Sasuke would likely be filled with rage. He and Naruto had a complex relationship, and Naruto's death at Sakura's hands would make Sasuke seek revenge or at least demand answers. Kakashi, on the other hand, would be deeply saddened. He was Naruto's teacher and would probably feel a sense of failure.
In some fanfictions, there could be an alternate reality or a time - travel plot involved. Maybe Kakashi from the future accidentally goes back in time and in the chaos of trying to fix something, he ends up killing Naruto. This would completely change the course of the Naruto - verse. Characters like Hinata, who had feelings for Naruto, would be heartbroken. And the Akatsuki might take advantage of the situation, seeing the Hidden Leaf Village in a state of disarray without Naruto as their potential Jinchuriki and future hope.
It could be due to some external influence. For example, there might be a dark power or a curse that affected Sasuke's mind and control at that moment. Or perhaps he was under the influence of some sort of mind - control jutsu that made him act against his will and accidentally kill Naruto.
One possibility could be that Jiraiya is under the influence of a powerful enemy's jutsu that makes him lose control of his actions and accidentally attacks Naruto. This could lead to a great deal of guilt and self - loathing for Jiraiya later.