In fanfiction, it could be that the Third Hokage favored certain ninja over Naruto for no good reason. For instance, giving more resources and opportunities to others while Naruto had to struggle on his own. This unfair treatment might be a reason for Naruto's hatred.
A possible event could be that the Third Hokage was involved in some sort of cover - up related to Naruto's past. Say, if there were secrets about Naruto's family that the Third Hokage was hiding, and Naruto found out about these secrets in a way that made him feel like the Third Hokage was just using him. This discovery could fuel his hatred in fanfiction.
In the fanfic, it could be that Kami has a conflict with one of Naruto's allies, and Kami's actions against that ally make Naruto angry and lead to his hatred. For example, if Kami tries to punish Naruto's friend for no good reason, Naruto would not be happy with Kami at all.
Perhaps the hunters always interfere in Percy's adventures. They might show up at the wrong time and disrupt his plans. For instance, Percy was about to rescue someone, and the hunters came and took over, but in a way that put the person in more danger. This would make Percy resent them. Also, if the hunters have some internal politics that end up affecting Percy negatively, like they accuse him of something he didn't do just to protect their own reputation, that could lead to his hatred.
One possibility is that Hestia took away something important from Percy in the fanfiction. It could be a magical item or a chance to achieve a great goal, which made Percy resent her.
Maybe he got into a big fight with his friends during Christmas. It could be that they had different ideas about how to celebrate or some misunderstanding that made him sour on the holiday.
One event could be if Tsunade refuses to help Naruto save one of his friends. Naruto is very loyal to his friends, and if Tsunade stands in the way of him rescuing someone dear to him, it might cause him to hate her. For instance, if she thinks the mission is too dangerous and not worth the risk for the village, but Naruto sees it as a matter of life and death for his friend.
He may dislike it because he feels that it undermines the real struggles and achievements of the ninja. In fanfiction, things might be made too easy or unrealistic. He has fought tooth and nail for everything in his ninja life, and seeing it made into something that doesn't represent that in fanfiction would make him hate it.
His relationship with Jiraiya could be affected. Jiraiya has a strong connection with the toads, and if Naruto hates toads, it could create a rift between him and his mentor. This could lead to Naruto losing an important source of guidance and support in the fanfiction.
One reason could be jealousy. If in the fanfic, Sakura gets closer to someone else Naruto cares about, like Sasuke in a different way than he expected, it might make him hate her. For example, if Sakura teams up with Sasuke against Naruto's wishes in the fan - created plot.
One consequence could be that he leaves the village. He might go off on his own to find a new place where he feels accepted and can start anew.
One possible reason could be jealousy. Maybe in the fanfiction, Jiraiya gives more attention to another character, and Naruto, being young and impulsive, starts to hate him for it.