One possible reason could be jealousy. Maybe his brother has some special abilities or gets more attention that Naruto feels he should have. For example, if his brother is really good at a certain jutsu and is always praised for it while Naruto struggles, that could lead to resentment.
The story could end with Naruto realizing his hatred was misdirected. After some big event or revelation, he discovers that his brother and sister actually care for him deeply. They all reconcile and become a closer - knit family.
One possible reason could be that his family in the fanfiction might be cruel or neglectful towards him. Maybe they constantly put him down or don't support his dreams and ambitions, which would lead to his hatred.
Percy might be initially confused. He may not understand why his half - brother has such strong negative feelings towards him. He could try to talk to him to resolve the issue, thinking it's just a misunderstanding.
It completely flips the typical mother - son relationship. Instead of the usual love and support, there's tension and hostility. It can make Harry more of a rebel, going against his mother's wishes or values.
One possible reason could be a personality clash. Harry might have a very different personality from his mother. He could be more adventurous and she might be overprotective, which could lead to resentment on his part. Fanfiction often takes the characters and twists their relationships in unexpected ways.
The plot might start with Harry gradually realizing his family's true nature and starting to distance himself from them. As he pulls away, there could be a lot of conflict and drama within the family.
One possible reason could be the way the Dursleys treated him. They were cruel, made him live in a cupboard, and constantly belittled him. This kind of mistreatment over the years could easily lead to hatred. Another aspect could be the lack of connection to his wizarding family that he was not aware of for a long time. He might feel angry at his parents for leaving him in such a situation without any real protection from the Dursleys.
On the other hand, this hatred could also fuel his growth in a different way. He might train harder to prove that he is better than his brother, or to get revenge. This could lead to a more powerful and perhaps more complex Naruto, as he battles with his inner emotions while also striving for power and recognition.
In some fanfiction, Harry might hate his brother due to jealousy. Maybe the brother gets more attention from their parents or is more successful in certain areas, like magic if it's in a Harry Potter - inspired fanfic. This can create a rift between them.