One possible reason could be extreme guilt. Maybe he fails to protect his friends in a major battle and feels so responsible that he can't bear it. For example, if Sasuke were to die because Naruto couldn't reach him in time, Naruto might sink into such deep guilt that he sees no way out but death.
Perhaps the village he loves so much turns against him. If the villagers, who he has always tried to protect, start to fear and shun him due to some false accusations or misunderstandings. Naruto, who values the bonds with the village so highly, might feel so betrayed and alone that he chooses to kill himself.
Another reason could be the loss of his ninja way. Suppose Naruto is somehow forced to go against his principles, like being made to use a dark and forbidden jutsu that goes against everything he stands for. This moral conflict could be so great that he decides to end his own life.
It could also be due to a sense of isolation. If all his friends turned against him for some reason, perhaps being wrongly accused of a heinous crime. Naruto, who values his bonds deeply, might find such a situation unbearable and choose to end his own life.
There are several reasons. He might be afraid of getting too close to people because he's lost someone important and is scared of more loss. It could also be that he's discovered some new power within himself that he doesn't understand and is worried about hurting others if he gets too involved. Additionally, there could be some political intrigue in the village. Maybe there are some people who are plotting against him and he's keeping to himself to avoid being a target or putting others in danger.
In this kind of fanfic, one possible reason could be unrequited love. Naruto might have felt so hopeless about his feelings for Sakura never being returned that he saw no way out. He had always been in love with her, and if she continuously rejected him and perhaps even chose someone else, it could have broken his spirit to the point of self - destruction.
One possible reason could be the loss of his friends. Naruto values his friends deeply, and if he were to lose someone like Sasuke again, it would be devastating. Another reason might be the pressure of being the Hokage. The responsibilities are huge, and he might feel like he's not living up to the expectations of the village. Also, memories of his lonely childhood could resurface, making him feel sad and alone, leading to him crying himself to sleep.
Maybe for shock value. Some writers want to create a story that stands out and goes against the norm. By having Naruto kill villagers, it's a huge deviation from the original story and can grab readers' attention.
Yes, such fanfictions exist. In these fictional scenarios, Naruto might kill himself because of Sasuke for several reasons. Naruto's whole life has been centered around Sasuke to some extent. If he feels that he has failed in his mission to bring Sasuke back to the right path and that the situation has become so dire that the only way to atone for his 'failure' is through his death, he might do it. Also, if Sasuke were to cause harm to someone Naruto holds dear, like the village or his other friends, and Naruto feels powerless to stop him in any other way, he could choose to end his own life as a last resort.
One possible reason could be the burden of his past. Naruto has faced a lot of hardships and the memories might be overwhelming him, making him want to be alone. For example, the loss of his master Jiraiya might have left a deep scar, and he might feel that isolating himself is the only way to deal with that pain.
Maybe he feels a sense of guilt. For example, if something bad happened in the village that he blames himself for, even if it wasn't really his fault. He could exile himself as a form of self - punishment. He has always been very hard on himself when it comes to the well - being of the village and his friends.
Some creators might want to explore a darker side of Naruto's character. Maybe they're tired of the always - positive and heroic Naruto and want to see what he'd be like if pushed to the extreme.
Some people might create it just for shock value. They want to get a reaction from the Naruto fan community by presenting such a radical and against - the - norm idea.