Perhaps in the fanfiction, some sort of dark magic or curse has been placed on Naruto. This could be messing with his mind and causing him to act in an insane manner. It could be a way for the author to introduce a new and dark plot element to the story. Another cause might be that he has been betrayed by those he trusts the most, which shatters his mental state.
In some 'naruto is mentally insane fanfiction', it could be due to long - term isolation. If Naruto is isolated from his friends and the community for a long time, it can have a negative impact on his mental health. He may start to lose touch with reality and develop signs of insanity. Also, if he discovers some dark secrets about his past or the ninja world that are too much for him to bear, that could be a contributing factor.
Well, it depends on how the fanfiction is written. Sometimes, the author might show Naruto being mentally insane through his erratic decision - making. He might abandon his long - held beliefs and values suddenly. But this is all within the realm of fan - created interpretations and not based on the canon Naruto series.
One possible plot twist could be that Naruto's mental instability causes him to misinterpret the Fem Kyuubi's intentions. Maybe he sees her as a threat when she's actually trying to help him. This could lead to a lot of internal conflict within Naruto and also create tension in their relationship.
Naruto, with his mental insanity, would likely develop in a very erratic way. His actions might be unpredictable as he tries to come to terms with his own mind and the presence of the Fem Kyuubi. The Fem Kyuubi, on the other hand, could start off as a more stable entity but gradually be influenced by Naruto's insanity. Maybe she becomes more empathetic towards his situation.
It could also be due to personal relationships. Perhaps some of his closest friends get caught up in their own lives. Like Sasuke might be on a long - term mission and not be there for Naruto, and Sakura might be too busy with her medical studies. So Naruto, who is used to having a support system, suddenly feels alone and neglected.
One possible reason could be the extreme pressure he faces in the fanfic. Maybe he has to deal with much more powerful enemies alone, which overwhelms his mental state. Also, if he experiences a lot of betrayals in the fanfic, like friends turning against him, it could lead to his mental instability.
It could also mean that Naruto's actions, which seem normal on the surface, are actually part of a more complex and insane plan in the context of the fanfiction. For example, his pursuit of becoming Hokage might have a much darker and more insane motivation that he keeps secret.
One possible plot could be that Naruto is constantly ostracized by the villagers even after all he has done for them. The years of being treated as an outcast and having his good intentions misconstrued finally push him over the edge. He starts to see the villagers as the real enemy and decides to use his power not for their protection but to make them feel the pain he has endured.
It could be that Naruto discovers some dark secrets about his past, such as the real reason his parents died or some hidden and malicious intentions of the village elders. This overwhelming knowledge could be too much for him to bear and push him over the edge into insanity. He would then start to act erratically, causing turmoil in the ninja world.
Madara Uchiha could also be a likely cause. If he somehow managed to return or had a long - planned scheme that involved breaking Naruto's sanity. He was a very powerful and devious foe who could use his knowledge of the ninja world and its powers to target Naruto's mental stability.
In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.