It could be a situation where Naruto discovers some dark secret about the Uchiha clan that he believes makes them a danger to everyone. For instance, if he found out they were planning a coup that would involve mass destruction, he might think that killing them was the only way to save the village and the rest of the ninja world.
Well, in this fanfiction scenario where Naruto kills the Uchiha clan, it would mean the end of the Sharingan bloodline in a very unexpected way. This would have huge implications for the ninja world. Without the Uchiha, there would be no potential for the Mangekyou Sharingan or the Eternal Mangekyou Sharingan. The balance of power in the ninja world might shift as the Uchiha were quite powerful. And Naruto himself would be a very different character. He's always been about protecting and saving people, so this act would make him a much darker and more complex figure, almost an anti - hero.
In this fanfiction scenario, Naruto killing the Uchiha clan would make him an outcast in Konoha. The village that once supported him would turn against him. This could lead to Naruto leaving the village and becoming a rogue ninja. He might then form his own group of allies who believe in his cause, whatever that might be in this altered story. It would also change how the Akatsuki views Konoha. Instead of targeting Sasuke, they might target Naruto directly as the one who eliminated the powerful Uchiha clan.
One way Naruto could save the Uchiha clan in fanfiction is by using his influence. Naruto is highly respected in the village. He could convince the village elders to re - evaluate their treatment of the Uchiha. He might show them that the Uchiha were being wrongly persecuted and that there was a way to co - exist peacefully. He could also use his power as a Jinchuuriki to protect the Uchiha from any external threats that might be a factor in their downfall.
If Naruto saved the Uchiha clan, the story of the Hidden Leaf Village would be rewritten. The history of the Uchiha's massacre would no longer be a dark stain on the village's past. The Uchiha would contribute their unique skills and abilities to the village's development. For example, their Sharingan abilities could be used for better security or in battles against common enemies. There would also be a change in the relationships between characters. People who were once enemies of the Uchiha might have to learn to work with them.
One possible motive could be revenge. Maybe Naruto was led to believe that the Namikaze clan was responsible for something terrible that happened to him or someone he cared about. Another motive could be power. If there was some sort of power within the clan that Naruto thought he could gain by eliminating them.
Well, one possibility is that Naruto would be trained in the unique Uchiha techniques. He might learn how to use the Sharingan in ways that no one expected. Also, he could gain a new family within the Uchiha clan, which would change his whole perspective on life and belonging.
In the fanfiction, it could be that Naruto has had some personal negative experiences with Uchiha clan members. He might have witnessed some Uchiha using their power to bully or oppress others. Naruto, being the kind - hearted person he is, would not tolerate such behavior and thus develop a hatred for the clan as a whole. This could also be related to the Uchiha's reputation for being a bit of an outcast clan, and Naruto might see them as not fitting into the harmonious village life he envisions.
His motive could be a false sense of loyalty to the village. If he believes that the Uchiha clan's actions, real or perceived, threaten the safety and stability of the village, he might think that massacring them is the only solution. This would show a more extreme and dark side of Naruto's character, where his loyalty to the village overrides his sense of right and wrong.
It could also be because of the Uchiha clan's perceived aloofness. The Uchiha might seem standoffish and exclusive, which could make Naruto, who values friendship and community, dislike them. They may have a reputation for being more concerned with their own clan's affairs rather than the well - being of the whole village.
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.