Naruto's insanity may also be seen in his broken relationship with his ninja way. He used to believe in never giving up and protecting his friends, but due to the neglect he's faced, he starts to question all of that. He might start using unethical means to achieve his goals. Instead of using his usual positive and friendly approach, he could turn to more violent and underhanded tactics, which is a complete departure from the Naruto we know.
Well, he might start having erratic mood swings. One moment he could be extremely withdrawn, not talking to anyone. Then suddenly, he might lash out at his friends for no real reason, like when Sakura tries to approach him to see if he's okay, he just yells at her and tells her to leave him alone.
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.
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.
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.
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.
The main antagonist could be Naruto himself in a sense. Since he has gone insane, his actions are now unpredictable and harmful to the people around him. His former friends, like Sakura or Kakashi, might have to step up and try to stop him, seeing him as the new threat.
The Akatsuki as a whole could be the opponents. Imagine Naruto facing multiple Akatsuki members at once in a fight. The combined threat of their different powers and their goals of capturing the tailed beasts could be overwhelming for Naruto. They might use underhanded tactics, like threatening to kill captives or using mind - control techniques on Naruto's friends. All of these things could make Naruto go insane as he battles against them, trying to protect what he loves.
One reason for this in fanfiction could be to give Naruto a different kind of arc. Instead of the somewhat positive neglect in the original (where it was more about being alone but still having some hope), here it's a full - blown abandonment. And his near - death experience can be used to show how fragile he really is, despite his strong - willed nature in the main series.
One possible plot could be that Naruto is pushed over the edge by the constant betrayal he faces. For example, if the village he has always protected turns against him due to false accusations of being a threat. He might start using his powers in a more erratic and dangerous way, not caring about the consequences. This could lead to a showdown between him and his former friends who now see him as a menace.