In some cases, the writing style of the fanfiction might be the issue. If it's too focused on the heroes and the OC is just thrown in as an afterthought, it can make the OC seem out of place. Also, if the heroes are given all the interesting storylines and the OC is left with boring or unoriginal ones, it can lead to a sense of hatred towards the heroes within the context of the fanfiction.
Perhaps the concept of heroes in the fanfiction goes against the OC's values. For example, if the heroes use violence as a first resort and the OC is a pacifist, that could lead to a dislike. It could also be that the fanfiction portrays the heroes as always being right, and the OC, who may have a different perspective, is constantly marginalized.
One reason could be a divergence from the original friendship dynamic in the books. In the canon, Harry and Ron are good friends, and fanfiction that shows Harry hating Ron might go against what many fans consider the essence of their relationship.
Maybe it's because of the different power systems. Naruto has chakra and his ninja abilities, and Rias has her devil powers. If in the fanfiction these power systems are not properly integrated or if one overshadows the other in an unappealing way, Naruto could be depicted as hating Rias. Say, if Rias' devil powers are made to seem too overpowering and it goes against Naruto's sense of fairness in combat, it could lead to such a scenario.
It could be that the fanfiction simply doesn't meet Boruto's standards for what the Naruto universe should be like. He has grown up in that world and has his own ideas about how things should be. Fanfiction can be very hit - or - miss in terms of quality and faithfulness to the source material. If it's of poor quality or changes things too much, Boruto might hate it.
One reason could be a misinterpretation of their complex relationship. In the original story, Naruto actually cares deeply for Sasuke. But in fanfiction, the author might be exploring a 'what if' scenario where Naruto's frustration turns to hate due to Sasuke's repeated betrayals. Another reason could be to create a more angsty or dramatic plot. By having Naruto hate Sasuke, it can introduce new conflicts and character developments that are different from the canon.
It could be that the fanfiction creates a forced or contrived situation. Naruto is a character known for his determination and positive energy. A plot where he hates his sister might not fit well with the overall Naruto universe. It might be a way for the author to create drama in a rather unappealing way that doesn't resonate with the essence of the Naruto character.
Maybe the fanfiction portrays Hinata in a way that doesn't align with her true character. For example, it might make her overly aggressive or weak in a way that doesn't fit the original concept.
One reason could be that it goes against the core themes of Naruto's character development. Naruto was always longing for family and his journey was about finding his place in the world which included his family roots. Fanfiction that has him hating family might seem out of character.
One possible reason could be that the original Naruto story has a very distinct and carefully crafted world and character arcs. Fanfiction might distort these in ways that seem disrespectful to the source material, making Naruto, as a character concept, seem to 'hate' it. For example, some fanfictions may change Naruto's core values or relationships in a way that goes against the essence of the character.
Well, it could be that the fanfiction is trying to show Ginny's jealousy or resentment in a new light. Perhaps she's jealous of Harry's fame or the attention he gets. Or maybe the writer is simply exploring a 'what if' scenario where Ginny and Harry's relationship took a different turn from the start. In the original story, they end up together, but fanfiction allows for alternative storylines, and this could be one of them where Ginny hates Harry for various reasons that might be related to their different personalities, goals or the pressure of the wizarding world around them.
Well, maybe in this fanfiction Naruto has some clashing values with Makarov. Naruto values loyalty and friendship highly, if Makarov somehow goes against these concepts in the fanfic, Naruto could hate him.