Perhaps Harry has seen the worst in people too often. He might have witnessed so much greed, envy, and cruelty among those around him that he has lost all faith in humanity and thus hates everyone. He could be in a situation where he feels completely alone and that no one truly understands him.
Dumbledore could also be an important character. He might be the one who tries to figure out what's wrong with Harry and tries to reverse this hatred. Another character could be Snape. He might see this as an opportunity to make Harry's life even more difficult or, on the other hand, might be secretly trying to help him get over this hatred.
Well, this kind of fanfiction might be an interesting take on the character. It could explore a darker side of Harry Potter that we don't often see in the original series. Maybe the author wants to show how all the pressure and trauma he has endured could lead to a more cynical view of those around him.
There could be his friends like Ron and Hermione. Even though he hates everyone, they would still be important characters as they might try to figure out what's wrong with Harry and how to bring him back from this state of hating everyone.
Maybe Harry has had some really bad experiences that made him bitter towards everyone. For example, he could have been betrayed by his friends multiple times.
Perhaps the plot could revolve around a magical accident or curse that makes Harry lose his ability to feel positive emotions towards others. As a result, he starts to hate everyone. He begins to act out against the people he once cared about, causing chaos in Hogwarts and the wider wizarding community. His friends would be shocked and try to find a way to reverse the effect, but Harry would resist their attempts.
It could be Naruto himself. Since he hates everyone, his actions against the people he once cared about could make him the antagonist. He might start attacking the Hidden Leaf Village or other ninja villages, making him the main source of conflict.
Perhaps Naruto gets corrupted by some dark force. It changes his personality completely. He might start seeing all the people in the Hidden Leaf Village as the source of his problems. He could remember all the times he was ostracized as a child and now blames everyone for it. So in the fanfic, his hatred for everyone grows as he views the world in a new, darker light.
Well, some fanfic writers might explore such a concept for the sake of creating an alternative storyline. Maybe they want to see how the dynamic between Harry and Hermione would change if there was conflict or hatred, adding a new and unexpected twist to the well - known relationship.
Maybe the writer wants to create a unique plot twist. In some fanfics, authors like to deviate from the original relationship to explore different character dynamics.