Initially, Naruto could be filled with anger. He might feel like all his efforts to protect the village and be accepted were in vain. However, his strong will and belief in himself would likely kick in. He could decide to train even harder, not just to prove his innocence but also to become strong enough to face whatever force was behind his betrayal. He might also start to question the values of the village he once held so dear.
Perhaps there was a dark secret about Naruto's origin that was revealed in a way that made others fear him. For example, if it was discovered that he had some connection to a feared enemy or an ancient evil. This could lead to his betrayal as people's fear overrode their previous loyalty to him. Also, jealousy among his peers could be a factor. If Naruto was constantly getting stronger and more popular, some might have become envious and were easily convinced to turn against him.
It could be a fellow student who was secretly jealous of Harry all along. Maybe someone like Draco Malfoy. He had always been at odds with Harry and in a fanfiction, he could be pushed to the point of betraying Harry for his own self - centered reasons. Another possibility is a teacher. A teacher who was influenced by the dark side or had some hidden agenda could betray Harry. For example, if there was a teacher who was secretly working for Voldemort or a new dark wizard.
It could be a rogue ninja from another village who has a vendetta against Naruto. This ninja might have set a very elaborate trap that made it seem like Naruto was killed and left his body in a place where his friends would find him and think he was dead.
He might survive through his own sheer willpower. Naruto has always been a tenacious character. So, even when left for dead, his determination to live and achieve his dreams could keep him alive against all odds.
Iruka Sensei could also be a candidate. He was one of Naruto's first supporters and has a deep connection with him. He would probably risk a lot to save Naruto.
One possible plotline could be Naruto finding unexpected allies during his time left for dead. Maybe some characters who previously had no connection to him come to his aid out of a newfound sense of justice or a change of heart.
Another plotline might involve Naruto's inner struggle and growth. He could discover new powers or a deeper understanding of his ninja way while on the verge of death. He might have to rely solely on his wits and the training he's had in the past to survive. For example, he could use his shadow clones in more creative and strategic ways to defend himself and find a way back to his village.
Perhaps there was some sort of dark influence or mind - control at play. An enemy might have manipulated Kakashi's mind in such a way that he believed Naruto was a threat and thus left him for dead, not realizing his true self.
One possible reason could be a misunderstanding. Maybe they were misled by false information or an enemy's trickery that made them think Naruto was a threat at that moment. Another reason might be extreme pressure or a life - or - death situation where they thought sacrificing Naruto was the only way to save the rest of the team or complete a crucial mission.