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.
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.
Perhaps they have wronged Naruto in some way in the fanfiction. They could have betrayed him, like telling his secrets to others or getting him into trouble on purpose. This would lead to Naruto developing hatred towards them.
Some writers might want to explore a different side of Naruto. Maybe they are tired of the same old positive - friendship - based stories and want to create a more edgy or dark take. They could be exploring what would happen if Naruto had a falling out with his friends due to some external factor like a power struggle or a betrayal that he wrongly perceives.
One possible reason could be a misunderstanding. Perhaps Naruto misinterprets some of his family's actions or words. Maybe he thinks his family didn't care about him as much as he thought they should. In fanfiction, authors often play with the idea of miscommunication and how it can lead to strong negative emotions like hatred. Also, if there are other characters in the fanfiction who influence Naruto's view of his family, say by spreading false rumors or manipulating him, that could contribute to his hatred.
It can be quite divisive. Some fans might be extremely offended as Naruto is a beloved character known for his positive traits. This type of fanfiction goes against the established character arc and values. It could make some fans angry that someone would distort Naruto's character in such a way.
It could be that the 'mother love' in the fanfiction goes against Naruto's own ideals or his perception of family. Naruto grew up without his parents, and if the fanfiction presents a mother love that is forced or not in tune with his experiences, he might hate it. He has his own understanding of love and family through his friends and the village, and this fanfiction concept could be a jarring deviation from that.
One way could be through Naruto feeling neglected by his family. Maybe in this fanfiction, his parents were alive but were always too busy with village affairs, leaving Naruto feeling alone and unloved. This could lead to his hatred as he grows up seeing other families being more present for their kids.
One possible reason could be that Naruto feels abandoned. His father was not around during his childhood, and he had to face a lot of hardships alone. Growing up without a father figure, he might have developed resentment.