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.
It could be that Harry has a power within him that he can't control and he blames everyone around him for his situation. He might think that they are either trying to use him or are afraid of him, thus leading to his hatred.
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.
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.
One possible plot could be that Harry has been repeatedly betrayed by those he trusted, all of whom are humans. This string of betrayals causes him to gradually develop a hatred for all humans. He might then go on a journey to isolate himself from human society and seek revenge on those who wronged him.
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.
It's possible that the plot could start with some kind of mind - control or brainwashing of Harry. Then, he would start to spread hatred towards Mudbloods in Hogwarts. This would cause a great divide among the students. There could be battles between those who support Harry in his new - found (but false) beliefs and those who oppose him. But again, this is a very dark and untrue - to - the - original - spirit plot that goes against the core values of acceptance and friendship in the Harry Potter series.
Perhaps in this fanfiction, Naruto discovers some dark secrets about the village. Secrets that make him feel like he's been used all along. Like the village was only using him as a weapon against their enemies. This realization could make him resent everyone in the village, thus hating everyone.
Well, it could be that Percy has had some really bad experiences in his life that made him bitter towards everyone. Maybe he was betrayed by friends in the past, so now he just can't bring himself to trust or like anyone.
In such a fanfiction, it could be that Harry Potter has been corrupted by a powerful and malevolent force. This force might have warped his sense of right and wrong. It could also be that the author wants to explore the extreme consequences of power and how it can turn a hero into a villain. For example, if Harry Potter was constantly taunted and mistreated by the wizarding community despite all he has done, he might reach a breaking point and lash out at everyone.
Well, perhaps in this fanfiction, Harry has been through so much that he decides to isolate himself from everyone. He might be tired of the constant danger and the expectations placed on him. So he shuts out his friends like Ron and Hermione, and just focuses on his own inner journey, maybe to discover some new magic that he believes can only be found in solitude.
One possible reason could be that the 'light' in the fanfiction represents something he associates with strict rules or a false sense of goodness. Maybe it has been used to manipulate people, and Harry, being perceptive, sees through it and thus hates it.