One reason could be years of built - up resentment. The villagers have treated Naruto poorly for so long, constantly shunning him and making him feel like an outcast. In fanfiction, this could reach a boiling point where he just snaps. For example, if someone in the village made a particularly cruel comment about his parents, it might be the last straw.
In fanfiction, Naruto might snap at the villagers if he witnesses them being unjust to someone else he cares about. He has a strong sense of justice, and seeing the villagers' unfair treatment of a friend or ally could push him over the edge. He would not stand for that kind of behavior, especially if it reminded him of how they treated him in the past.
One reason could be miscommunication. Maybe Sakura said something that Naruto misinterpreted as an insult to his dreams or abilities. For example, if she made a comment about his progress as a ninja in a way that came off as dismissive.
One possible reason could be fear. Naruto has the Nine - Tails inside him, and the villagers might be so afraid of the power of the Nine - Tails getting out of control that they decide to take extreme measures. They might see Naruto not as a person but as a potential threat that needs to be eliminated.
Well, one reason could be Sakura's constant nagging about Sasuke. Naruto has his own feelings and goals, and if Sakura is always pushing the Sasuke matter in a way that dismisses Naruto's feelings, it might make him snap. For example, if she keeps asking him to bring Sasuke back without considering how hard it is for Naruto.
It could be a situation where Inari is interfering with Naruto's mission in the fanfiction. Naruto is very dedicated to his missions and protecting his friends. If Inari were to do something that put Naruto's mission or friends at risk, Naruto would likely snap at him.
One possible reason could be the way the villagers treated him as a child. They ostracized him because he was the host of the Nine - Tails. He was often alone and looked at with fear and disdain, which could have gradually built up his hatred.
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.
One reason could be stress. Izuku might be under a lot of pressure to keep up with his training and the expectations of being a hero. Class 1A's constant high - energy and sometimes chaotic behavior could push him over the edge. Another reason could be misunderstandings. If they misinterpret his actions or motives, he might snap in frustration. For example, if they think he's showing off when he's actually just trying to help.
One possible reason could be the long - standing emotional neglect Shinji has endured from Gendo. In the Evangelion series, Gendo is often distant and cold towards Shinji, and in fanfiction, this could build up to a point where Shinji snaps.
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.
Maybe the villagers were being ungrateful for his past sacrifices. Minato was the one who saved the village from the Nine - Tails' attack. If they started to forget his deeds or were misusing the peace he had brought, he would be angry. He might have thought that his efforts were in vain if the villagers didn't uphold the values he fought for.