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.
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.
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.
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.
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.
One element that makes it funny and insane is when the author takes a normal Naruto concept and blows it way out of proportion. For instance, if the concept of chakra is turned into something like 'chakra candy' that ninjas have to eat to gain power. And when the fanfiction has a lot of physical comedy, like Naruto constantly tripping over his own feet while trying to perform a super - important jutsu. Also, when there are crossovers with really unexpected universes, like Naruto meeting the characters from a Looney Tunes world. This creates a really wacky and fun dynamic.
In this type of fanfiction, the story could also focus on how the village might react differently to Naruto. Without Kushina's presence, which could potentially have been a positive influence, Naruto might face more hostility. His struggle to fit in and gain acceptance could be more arduous, and this could be what drives him to the brink of insanity. Maybe he would develop a more jaded view of the world and the people in it.
Well, there could be a 'naruto insane naruto fanfiction' where Naruto teams up with his enemies in a very unexpected way. Maybe he forms an alliance with Orochimaru not for power, but to save the entire ninja world from a new, greater threat. They would have to put aside their differences and work together, which would be a really insane concept considering their past relationship.
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.
The alternate storylines are also really cool. Maybe Naruto becomes a villain or there are completely different power - ups and battles. It gives a fresh take on the Naruto universe. Some fanfictions might even introduce new characters that blend well with the existing ones.