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.
Another way is to create a conflict where the OC has to work with the heroes despite hating them. This can lead to some interesting character development for both the OC and the heroes. The OC could gradually learn to see the heroes in a different light, or the heroes could realize that they have been wrong in some ways. You can also add some mystery to the story. Maybe the OC has a secret power that the heroes need, and this forces them to interact. This power could be related to something that the heroes have been searching for, but they didn't expect it to come from someone who hates them.
It could be that the OC creator simply doesn't like the typical tropes or storylines often found in Rias fanfiction. For example, if the fanfictions tend to be very formulaic or over - use certain clichés, the OC might be a symbol of a break from that. Also, perhaps the OC has a specific backstory or purpose that is at odds with the general narrative of Rias fanfiction.
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.
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.
One reason could be jealousy. Maybe Snape gets jealous of Lily's other relationships or her popularity at Hogwarts. Another could be a fundamental difference in their beliefs or values that grows over time into hatred.
Well, in the 'naruto hates zeus fanfiction', it could be due to cultural differences. Naruto's world has a specific Japanese - inspired culture, while Zeus is from Greek mythology. Naruto might resent Zeus for representing a foreign and perhaps overpowering entity that doesn't fit into his understanding of the world.
It could be that the fanfiction writers wanted to explore a more complex and darker side of relationships. They might have written scenarios where Aang hates Katara due to jealousy, perhaps over her friendship with another character. This would add an interesting twist to the original relationship dynamics between 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.
Maybe in that fanfiction, McGonagall has some actions that Harry misinterprets. For example, she might be too strict with him in a way that he can't understand at that moment.
Maybe Naruto feels overshadowed by the Rookie 9 in the fanfiction. In the original series, Naruto has his own journey and development. If the fanfiction misrepresents his relationship with the Rookie 9 in a negative way, like making them seem better than him all the time, he could hate it.