Perhaps in the fanfic, Naruto is under the influence of a powerful and malevolent genjutsu (illusion technique). This genjutsu makes him see only the worst in his life, like all his achievements being for naught and everyone he loves actually hating him deep down. Unable to break free from this illusion that constantly tortures his mind, he may consider ending his life.
In such a fanfic, it might be due to a false accusation. Suppose Naruto is wrongly accused of a heinous crime like mass murder in the village. Everyone turns against him, and he has no way to prove his innocence. This overwhelming sense of injustice and isolation could drive him to suicidal thoughts. Also, if he discovers some dark secrets about his past, such as his parents' real cause of death being related to some sort of betrayal he unknowingly caused, it could break his spirit.
One possible reason could be extreme loneliness. Naruto has always faced some degree of isolation in the original story. In a fanfiction, this could be exaggerated to the point where he feels there's no way out and becomes suicidal. Another reason might be the failure to protect his loved ones. If in the fanfiction, he fails to save his friends or the village multiple times, the guilt could be overwhelming.
Perhaps he has faced repeated failures in protecting his loved ones. If he constantly blames himself for not being strong enough to keep his friends safe, it could lead to self - loathing and suicidal tendencies. For example, if Sasuke leaves again and Naruto feels it's his fault, it might push him to the edge.
One reason could be exploring dark themes in the Harry Potter universe in a more extreme way. Some fans might be interested in delving into the psychological struggles that could potentially exist in that world, even if they are very dark. Another reason could be a form of catharsis for the authors, who may be dealing with their own internal issues and using the fanfic as a way to express their feelings in a fictional context.
It could be that the writer wants to explore the consequences of extreme distress in the context of the story world. Maybe they want to show how Percy's friends would react to such a situation, or perhaps they are using it as a plot device to create a very emotional and impactful story arc. Also, it might be a way to raise awareness about mental health issues in a fictional context, although this is a rather extreme way to do it.
Perhaps a dark power corrupts him. In the fanfiction, there could be some sort of dark chakra or evil entity that latches onto Naruto. This entity could slowly eat away at his emotions, starting with his heart, making him gradually lose his ability to feel empathy and turn heartless.
One reason could be curiosity. Naruto might be curious to see how fans imagine different scenarios in the Naruto Shippuden universe. Maybe he wants to see how others view his friends and his own adventures.
Yes, there might be such fanfics. One possible reason could be exploring alternative storylines. Writers might want to break the traditional view of Naruto's loyalty to Konoha to create more complex and unique plot twists. For example, they could be imagining a scenario where Naruto discovers some dark secrets about Konoha that make him question his loyalty.
Well, at first, Naruto would probably start to withdraw from his normal activities. He'd stop going on missions and spending time with his friends. Then, he might start having self - destructive thoughts more frequently. As the story progresses, his friends might notice something is wrong, but Naruto would be reluctant to talk about it. Eventually, if no one can reach him in time, he could attempt something dangerous to himself.
Maybe Naruto discovers that the village he has been trying so hard to protect has been using him all along. He might find out some dark secrets about the Hidden Leaf, like they were involved in something immoral and were just using his power, which would shatter his idealistic view of the village and leave him heartbroken.
Another possibility is jealousy. Maybe in the fanfic, Sakura shows more interest in another character, like Sasuke perhaps, and Naruto, who has his own feelings for Sakura, snaps out of envy. It's a common trope in fan - made stories to play with the characters' emotions like this.