One reason could be his past experiences of being bullied and looked down upon. In fanfiction, this might push him to overcompensate during the Chunin Exams. He wants to show everyone that he's not the weakling they thought he was, so he becomes brutal in his fighting style. Another reason could be the influence of some dark power or a mentor who encourages him to be more aggressive.
In fanfiction, Naruto may be brutal in the Chunin Exams because of the high stakes involved. He might feel that winning at all costs is necessary to achieve his dreams, like becoming Hokage. Also, the intense competition and the pressure from his village could lead him to be more cut - throat. He may think that being brutal is the only way to stand out among the other strong contenders. Additionally, if he has witnessed some unfairness in the exams before, he could be determined to take matters into his own hands in a rather brutal way.
Well, in certain fanfiction about the Chunin Exams, Naruto could be seen as brutal because he might take advantage of his opponents' weaknesses in a rather ruthless way. He may not give them a chance to recover or adjust their strategies. Say, if an opponent is vulnerable after using a big move, Naruto might immediately strike with a powerful counter - attack instead of waiting for a fair rematch. This makes him seem brutal as it goes against the more sportsman - like nature of the regular Chunin Exams.
The interference of external forces. In fanfiction, there could be some hidden enemies or manipulators who are trying to make Naruto lose control during the Chunin Exams. They might be spreading false rumors about him or creating situations that are designed to break his spirit, such as framing him for cheating or something similar.
One possible reason could be that Naruto realizes the Chunin Exams have become too dangerous and he doesn't want to risk his life or the lives of his friends. Maybe he sees the dark side of the competition, like how some contestants are willing to cheat or harm others to win.
One reason could be if Sasuke was somehow possessed by a powerful and evil force during the Chunin Exams. Naruto might see no other way to stop this threat and be forced to kill Sasuke to protect the village and others. Another possibility is a huge misunderstanding. For example, if Sasuke was framed for a crime he didn't commit and Naruto was misled into believing he was a danger.
Another reason could be related to his personal goals. Naruto might have something else in mind that he believes is more important than taking the Chunin Exams at that moment. For example, he could be focused on finding out more about his family or some mystery in the village. He may also not want to participate because he doesn't want to be part of the traditional system that he might view as unfair or too rigid. He's always been a bit of an outlier, and the Chunin Exams represent a very structured and competitive event that might not appeal to his free - spirited nature.
A possible plot could be that female Naruto is targeted by a group who don't want her to succeed in the Chunin Exams. They try to sabotage her at every turn, but she overcomes their obstacles with her determination and the help of her friends. Another plot could be that she discovers a hidden power during the exams that she didn't know she had, which gives her an edge over the other competitors.
Perhaps in this fanfiction, Naruto is fatally injured in a fight during the Chunin Exams. He might sacrifice himself to save one of his friends. This would cause a great outpouring of grief from his friends and mentors. The village would also be in shock, and there could be a change in the power dynamics as his absence leaves a void.
One possible consequence could be a huge shift in the power dynamics among the ninja villages. Since Naruto was seen as a potential strong force, his death might make Konoha more vulnerable. Also, his friends like Sasuke and Sakura would be deeply affected. Sasuke might be driven further into his quest for power in a more reckless way out of revenge or a sense of loss. And Sakura would likely be devastated, perhaps changing her path from a healer to someone more focused on vengeance as well.
One plotline could be that Naruto's godlike powers attract the attention of powerful enemies outside of the village. These enemies try to either capture or kill him during the Chunin Exams. Naruto has to use his newfound powers to protect not only himself but also his friends and the village.
Perhaps an injury could cause him to quit. If he gets seriously hurt in one of the early rounds and realizes that continuing could lead to permanent damage or even death, Naruto might choose to step down. He's brave, but not foolish. He also has dreams of becoming Hokage and if he thinks that by continuing in the Chunin Exams in an injured state he could jeopardize his long - term goal, he would rather quit and live to fight another day.