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.
One great source of humor in Naruto fanfiction is when the characters are put in out - of - character situations. For example, imagine Naruto suddenly having to cook a five - star meal for a visiting dignitary. His clumsy attempts at cooking would be hilarious. He might mix up the ingredients completely, like adding salt instead of sugar to a cake batter.
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.
You can also create humorous crossovers. For example, mix Naruto with a fairy - tale world. Naruto could be the 'big bad wolf' in a twisted version of Little Red Riding Hood, trying to steal the 'ramen basket' instead of the food in the basket. This unexpected combination can bring a lot of laughs. Also, use witty dialogue. Let the characters make puns or sarcastic remarks. Like Sakura saying to Naruto, 'You're as sharp as a kunai made of rubber' when he does something stupid.
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.
One example could be a fanfiction where Naruto is neglected by his friends in the Hidden Leaf Village after a major event, and he starts to develop some rather extreme and uncharacteristic behaviors as a result. Maybe he becomes overly distrustful and starts to isolate himself, which is a far cry from his normal, positive self.
Well, in such fanfictions, one characteristic could be a very different take on the characters. Anko might be involved in a story where Naruto's 'insane' side (which could be interpreted in various ways like extreme power, a broken mental state etc.) is explored in relation to her. Maybe it shows a darker or more complex relationship than what is shown in the original Naruto series.
There are also 'insane Naruto fanfiction' stories that focus on alternate character relationships. For instance, a story where Naruto and Sasuke are not rivals but rather work together from the very start. This changes the entire arc of the original story and allows for new adventures and character developments that we wouldn't see in the canon Naruto series.