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.
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.
Perhaps it's related to some ancient ninja art or a hidden power within Naruto himself that gets awakened. In the Naruto universe, there are often latent powers waiting to be discovered. This Hercules - like strength could be tied to his Uzumaki heritage or some long - forgotten jutsu. He may initially struggle with this new power, not knowing how to fully utilize it without causing harm to himself or others. But as the story progresses, he learns to master it.
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.
Well, in fanfiction, this 'Naruto has strength of a hundred seal' could be a really cool concept. It might imply that Naruto has mastered a hundred different sealing techniques or has a special seal within him that gives him extraordinary strength. This could lead to all sorts of interesting plot developments, like him being able to defeat much stronger enemies or unlock new abilities that were previously unknown in the Naruto universe.
In such a fanfiction, Naruto could easily defeat any enemy. He might be able to single - handedly end all the wars in the Naruto world. For example, he could take on the Akatsuki by himself and quickly dismantle their plans without breaking a sweat.
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.