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 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.
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 possible plotline could be that Naruto is pushed to the brink during a fight against a powerful and sadistic enemy. Maybe this enemy has been targeting his friends one by one, causing Naruto to gradually lose his sanity as he tries to protect them. He might start using extremely violent and uncharacteristic jutsu, taking risks that he normally wouldn't. Another plotline could be that Naruto is under the influence of a dark genjutsu during a fight. This genjutsu twists his perception of reality, making him believe that his friends are actually enemies, and he goes insane trying to fight them off.
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.
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.
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.
Maybe it could be due to the loss of his friends. For example, if Sasuke died in a battle and Naruto couldn't save him, the grief might push him over the edge and into insanity.
I'm not entirely sure specifically what a 'naruto league insane fanfic' is about without reading it. But generally, it could be a fan - created fictional story within the Naruto universe that might have some really wild or extreme ideas, perhaps involving unique battles, unexpected character relationships, or crazy plot twists.
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.