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.
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.
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.
Some writers might create such fanfiction for the shock value. They want to break the established strong friendship in the original series to get readers' attention.
Yes, there might be some out there. Fanfiction is a vast and diverse world where writers explore all kinds of alternative scenarios. Some writers may create stories where Harry Potter hates Dumbledore for various reasons, such as feeling manipulated or betrayed by Dumbledore's actions. However, this goes against the general relationship portrayed in the original books, which is why it's an interesting and somewhat controversial topic in fanfiction.
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 impact is that it can divide the fandom. Some fans who love the original relationship between Harry and Hermione might be put off by such fanfiction. On the other hand, it can also attract new fans who are interested in seeing this new take on the characters.
Probably not as popular as the regular positive or canon - compliant Harry Potter fanfiction. Most fans love the characters and the world, so bashing everyone might not appeal to the general fanbase.
Perhaps Dumbledore is shown to have a secret alliance with Voldemort in the fanfiction. Harry stumbles upon evidence of this and is filled with rage and hatred. Or maybe Dumbledore had been manipulating Harry's emotions and actions from the very beginning to fit his own plans for the wizarding world, and once Harry realizes this, his hatred for Dumbledore grows. It could also be that Dumbledore has been hiding a powerful magical artifact that could have saved many lives but didn't, and Harry blames him for it.