One reason could be that in this fanfiction's unique plot, there is a prophecy or some ancient magic that has been misinterpreted. This misinterpretation leads Harry to think that Muggle - borns are somehow linked to an impending doom for the wizarding world. So, as a result, he harbors hatred towards them. It could also be that there has been a series of unfortunate events that he wrongly blames on Muggle - borns, like a magical accident that he believes was caused by their 'uncontrolled' magic.
In the original books, Harry is seen as a symbol of hope and acceptance. He fights against Voldemort who believes in pure - blood supremacy. But in the 'harry hates muggleborns fanfiction', Harry is on the wrong side of this equality issue. This fanfiction takes the character of Harry in a direction that is completely at odds with what we know from the original series. It also likely changes the relationships between characters. For example, his friendship with Hermione would not exist in the same way or might be completely broken in this fanfiction's universe.
Yes, there probably is some amount of it. However, in the official books, Hermione and Harry have a very strong bond. They support each other through countless challenges. Fanfiction like this may explore alternative character dynamics, but it's not the mainstream view of their relationship.
Yes. In the Harry Potter universe, the concept of hating 'mudbloods' is a form of bigotry. The original series promotes love, acceptance, and the fight against discrimination. A fanfiction with this title seems to go against the core values of the series.
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.
It could be for the sake of exploring an alternate personality for Harry. Writers might want to see how the story would change if Harry had a negative view of Muggles. Maybe they're exploring a darker side of the wizarding world where such prejudices exist among the main characters.
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.
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.
Perhaps in the 'harry hates dunbledore fanfiction', Dumbledore is shown as being involved in some sort of secret plot that goes against Harry's values. For example, if Dumbledore was secretly working with the dark side in the fanfiction, Harry would naturally hate him. Also, it could be that Harry has discovered some long - hidden truth about Dumbledore that completely changes his view of him, like a past misdeed that Dumbledore has covered up.
I'm not sure of a specific 'Harry hates needles' fanfiction off - hand. But you could start by looking on fanfiction platforms like Fanfiction.net or Archive of Our Own. Search for tags like 'Harry Potter' and 'hates needles' to narrow down the results.