This event in the fanfiction would likely change the future of the ninja world. The balance of power between the villages could shift. The Uchiha clan's legacy would be severely affected. It could also inspire new conflicts or alliances. For example, some of Sasuke's followers or those who believed in his cause might seek revenge against Naruto or Konoha. The fanfiction would then explore how Naruto deals with the consequences of his actions in this altered universe.
It would also affect Naruto's relationships with other characters. His friends like Sakura would be devastated. She has deep feelings for Sasuke. Naruto's own self - perception would be shattered. He has always seen Sasuke as his friend and rival, and taking his life would be a major blow to his moral compass. Moreover, it could lead to a power vacuum in the ninja world as Sasuke was a powerful shinobi.