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.
Several characters have met their demise in the MHA manga, but it's a bit complex to list them all precisely. Some of the notable ones include [mention specific characters if known].
I think it's hard to say for sure. There are a few characters who could potentially be the traitor, and it's all part of the mystery and suspense of the story.
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.