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.
In the 'naruto is left for dead by team 7 fanfiction', it could be that an outside force was controlling Team 7. This force could have been a powerful enemy who had the ability to manipulate their minds. They wouldn't be acting of their own free will. Another aspect could be that Naruto had made a sacrifice play himself. He might have pushed Team 7 away, making them think he was dead already so that they could escape a dangerous situation. And then there's the possibility that Team 7 was in a state of panic. When people are panicked, they might make hasty and wrong decisions, like leaving Naruto behind thinking he was already beyond saving.
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 sense of being undervalued. Maybe he felt that his teammates didn't fully appreciate his efforts during the Wave mission. For example, if Sasuke got most of the credit, Naruto might feel pushed aside.
He could survive through his own tenacity and willpower. Naruto has always been a fighter, so even when left alone, he might find the inner strength to keep going. Maybe he discovers some hidden power within himself that he didn't know before.
Perhaps in this fanfiction, Naruto has been influenced by some external force. It could be a dark entity that has corrupted his view of Team 7. He might see them as a hindrance to achieving some greater, more self - centered objective. Also, it could be that Naruto has had some sort of personal revelation about his past and blames Team 7, or certain members of it, for things that he now believes held him back in the past.
It could also be due to a major disagreement with Kakashi. Maybe Naruto has a different vision for how they should operate as a team or a particular mission that goes against his values, causing him to leave.
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.
One reason could be some sort of betrayal he feels. Maybe he discovers a secret that makes him think Team 7 was never truly his team. For example, if he found out that the village leaders had some ulterior motives for forming the team that involved using him.
One possible reason could be that Naruto might feel he doesn't fit in with the members of Team 7. Maybe he has some personal goals that he thinks he can't achieve while being on the team. For example, he might want to focus more on individual training to become stronger in a specific area like mastering a certain jutsu, and he believes being on a team would be a distraction.
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.
Perhaps Naruto quits Team 7 in the fanfiction because of a betrayal. He might find out that one of his teammates has been secretly working against him or has lied to him about something really important. This could shatter his trust in the team and make him want to leave. Or it could be due to an accumulation of small incidents. Like constantly being the one to get in trouble for the team's mistakes, or not having his opinions heard when it comes to team strategies. Over time, these little things can build up and make him feel that he's better off on his own.