It could be a misunderstanding. Perhaps Naruto was involved in an incident where his actions were misinterpreted by his brother. For example, Naruto might have accidentally caused harm to someone his brother cared about while trying to do something good, and his brother didn't get the full story and started to hate him as a result.
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.
Another reason might be related to family politics. In some complex family structures in the Naruto universe, there could be different factions. Naruto might be on the wrong side of the power struggle within the family. His views or actions could be seen as a threat to the traditional power structure of the family, so they hate him.
In this fanfiction, it could be that Naruto's parents were under some sort of mind - control. Some evil entity could have taken over their minds and made them feel hatred towards Naruto. Also, it might be related to a family secret or feud. There could be something in the family's past that made them believe Naruto was a stain on their family name, even though Naruto himself was innocent. It could also be a case of false information being spread around the village which made his parents believe lies about him.
One possible reason could be a misunderstanding. Maybe some false information was spread about his mother, making Naruto believe she had abandoned him. Another reason could be related to the secrecy around his mother. If he grew up feeling alone and not knowing about her, he might develop resentment.
In some fanfictions, Naruto might be hated by his friends due to a misunderstanding. For example, if there's a false rumor spread about him being a traitor. Another reason could be his sudden increase in power that makes his friends jealous. Maybe he accidentally harms one of them during a training session and they hold a grudge.
Perhaps there are family secrets or misunderstandings. His family might have some hidden agenda or past issues related to Naruto's brother that make them focus more on him. It could also be that Naruto's brother is seen as more compliant with the family's traditions and expectations, while Naruto is more of a free - spirit, which causes the neglect.
Naruto might also be determined to change his brother's mind. He would keep showing his good side, like his kindness and his strong will to protect the village. He could think that by being a better person, his brother will eventually stop hating him. He may even go through some tough training to prove himself not only to his brother but also to the whole village. And in the process, he may face a lot of difficulties and rejections from his brother, but he won't give up easily.
A possible reason could be jealousy. If Naruto had a sibling who was always favored by their parents, he might grow to hate his family. He would feel like he was always in the shadow of his sibling and that his family didn't care about him as much.
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.
One possible motive could be if his brother was somehow a threat to the village. Naruto is very loyal to Konoha, so if his brother was planning to destroy it or harm its people, Naruto might feel compelled to stop him at all costs.