One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
Revenge could be another factor. Suppose Sasuke blames Naruto for something that happened to his clan or his loved ones in the fanfiction's alternate storyline. He might seek revenge and end up killing Naruto. Also, a miscommunication or a huge misunderstanding between them could lead Sasuke to think that Naruto has betrayed him, causing him to take such a drastic step.
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 that Naruto feels he has become a threat to the village or his friends in some way. Maybe he has some sort of dark power within him that he can't control and he believes Sasuke is the only one strong enough to stop him. Another reason might be related to a prophecy or a greater threat that Naruto thinks his death can prevent, and he trusts Sasuke to carry out this difficult task.
One possible reason could be his thirst for power. He might see Naruto as an obstacle in his pursuit of greater strength. Maybe he believes that by eliminating Naruto, he can prove himself to be the stronger one and gain more recognition in the ninja world.
Maybe Sasuke goes too far in his pursuit of power and tries to harm someone Naruto cares about deeply in the Forest of Death. Naruto, in a fit of rage and to protect the person, kills Sasuke.
One reason could be if Sasuke was somehow possessed by a powerful and evil force during the Chunin Exams. Naruto might see no other way to stop this threat and be forced to kill Sasuke to protect the village and others. Another possibility is a huge misunderstanding. For example, if Sasuke was framed for a crime he didn't commit and Naruto was misled into believing he was a danger.
One reason could be to create shock value. By having two main characters kill another important character, it grabs the reader's attention. Another could be to explore a very dark and alternate version of the Naruto universe. Maybe the writer wants to show how far characters could be pushed in extreme situations.
It could be because of a dark curse. Suppose Naruto was cursed by an ancient evil. This curse slowly ate away at his sanity and made him view his friends in Team 7 as threats. As the curse took over more and more, he finally reached a point where he killed them. However, this would require a lot of backstory and character development in the fanfiction to make it somewhat believable.
It could also be due to a misunderstanding. Perhaps there is some misinformation spread about Sasuke in the fanfiction, like false rumors that he has a hidden and malevolent plan. Naruto, being someone who values their friendship deeply, would be scared at the thought of Sasuke turning into an enemy because of these untrue stories.
One possible motive could be if Sasuke was on a path of destruction that endangered the entire ninja world. Naruto, as the protector of peace, might see killing Sasuke as the only way to stop him. For instance, if Sasuke was about to unleash a powerful and uncontrollable jutsu that would wipe out many innocent lives.
One possible reason could be his deep connection with Naruto in the original story. Fanfiction might offer new and different perspectives on their relationship, which intrigues him. Maybe he wants to see how others interpret their bond, whether it's their friendship or the complex rivalry. Another reason could be that fanfiction allows him to exist in scenarios that didn't happen in the official storyline, giving him a chance to explore different aspects of his character and his interactions with other characters.