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.
It depends on your perspective. To some extent, Pecola shows signs of mental instability at the end, but it's also a reflection of the harsh circumstances she's been through and the lack of support around her.
No, Betty isn't insane. She's often shown as a kind, friendly, and sensible person in the comic series. Her actions and behaviors are typical of a normal character.
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.
Well, the themes play a big role. Themes like the loss of sanity, the breakdown of society, or the exploration of the darkest parts of human nature can make a horror novel disturbing and insane. For instance, if a novel delves into the idea of a person slowly losing their grip on reality and starting to do unthinkable things, it's both disturbing and shows a form of insanity. Additionally, the setting can contribute. A desolate and otherworldly setting, like an abandoned asylum or a haunted forest, can enhance the disturbing and insane aspects as it creates an atmosphere of dread.
Another plotline might involve Bella's insanity being a mystery at first. The other characters in the story gradually discover that she's not acting normal and start to investigate what's wrong with her. This can create a sense of suspense and mystery throughout the fanfiction, as the readers are also trying to figure out the cause of her insanity along with the characters.
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.