Perhaps in the fanfiction, Naruto has had some bad experiences with Sakura. Maybe she has let him down in crucial situations, like not believing in him when he needed it most. This could lead to his feelings of despise as he feels betrayed by her lack of faith.
One possible reason could be that the fanfiction misrepresents the true essence of Konoha. Naruto has a deep connection to Konoha, and if the fanfiction distorts its values, he might despise it.
One reason could be that Sakura is confused about her own feelings. Maybe she has feelings for someone else and doesn't know how to handle Naruto's obvious affection towards her, so she lashes out and ends up hurting him.
One possible reason could be extreme jealousy. Sakura might be so blindly in love with Sasuke that she sees Naruto as a threat to Sasuke's attention or her relationship with Sasuke. Maybe she thinks Naruto is getting in the way of Sasuke's return to the village or her attempts to win Sasuke back.
Well, Naruto could hate Sakura and Sasuke because in the NaruHina fanfiction context, Sakura might represent the past and all the times Naruto felt rejected by her. She always had eyes for Sasuke. And Sasuke, his actions were often self - centered and caused Naruto a great deal of distress. Naruto, as he falls for Hinata, starts to realize that Hinata is always there for him. Unlike Sakura, who is always pining for Sasuke. So, Naruto's feelings towards Sakura and Sasuke change to dislike as his relationship with Hinata blossoms.
One possible reason could be that he has been through so much trauma that he has numbed himself. For example, all the losses he has endured during his battles and in his personal life might have made him build a wall around his emotions.
One possible reason could be that Sakura realizes she has different goals or values that are not compatible with being around Naruto. Maybe she discovers a new path in life that doesn't involve him, such as focusing on her own medical ninja career without Naruto's chaotic lifestyle interfering.
Perhaps there was some sort of dark influence at play. In the world of Naruto, there are often dark forces and genjutsu (illusion techniques). It's possible that Naruto was under the influence of a genjutsu that made him see Sakura as a threat, so he punched her in self - defense without realizing it was an illusion.
One possible reason could be a misunderstanding. Maybe Naruto misinterprets some of Sasuke and Sakura's actions as being against him or the village. For example, Sasuke's quest for power might seem like a threat to Naruto, and he could misread Sakura's loyalty to Sasuke as a form of betrayal to their friendship.
Perhaps in this fanfiction's universe, Naruto's goals and ideals are completely different from Kakashi and Sakura's. Naruto may be aiming for a more radical change in the ninja world, while Kakashi and Sakura are more conservative. Their differences could lead to Naruto developing a sense of hatred towards them as they constantly oppose his ideas.
One reason could be that Naruto has always been more attracted to those who understand his outcast past better. Sakura doesn't really have that kind of connection with him as she comes from a more normal background. So there's a lack of that fundamental understanding between them which hinders any romantic feelings.