It could also be that Percy has a power or a secret that he's afraid others will find out. He's constantly on edge, worried that people are trying to get close to him just to take advantage of him. This paranoia makes him hate everyone around him, believing that they all have ulterior motives.
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.
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.
Obviously, Percy would be the main character. But there could also be other characters like his family members who might be trying to figure out why he has such a negative attitude. Maybe there's a best friend from his past who he now hates but who still tries to reach out to him.
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.
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 Percy finds himself in a world where everyone he knew has had their memories wiped about him. He might start by trying to figure out what caused this. He could go to his old friends' places, only to be met with blank stares. Then he might try to find a way to reverse the situation, perhaps by going on a quest to find a magical artifact or meet a powerful being. Along the way, he could face challenges alone, as he has no one to rely on, which would force him to grow and discover new abilities within himself.
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.
Perhaps Percy has received some bad news or a prophecy that makes him think that by ignoring everyone, he can protect them. So he distances himself from his friends and family, and the fanfiction could explore how they react to his sudden cold - shoulder. They might try to figure out what's wrong with him and how to bring him back to his normal self.
In a fanfiction like this, it could be that Percy's powers or abilities were starting to make others jealous. His friends might have felt overshadowed and, in a moment of weakness, turned on him. Also, there could be an outside influence, like a dark force that was manipulating people's minds to make them betray Percy.
Well, one reason could be that fanfiction often takes creative liberties that might distort the original relationship between Poseidon and Percy in the official story. It could be that Poseidon is a very complex character in the original, and fanfiction simplifies or misrepresents him in a way that makes it seem like he 'hates' Percy in those fictional interpretations.