Maybe in the fanfic, there was a huge misunderstanding. For example, perhaps the Weasleys were wrongly accused of something that endangered Harry's life or the lives of his loved ones. It could also be that there was some sort of dark magic at play that made Harry believe the Weasleys were his enemies.
In the fanfic, it could be related to a power struggle. If there was a powerful magical object or position involved, and the Weasleys were somehow in the way of Harry getting it. Or maybe there was a prophecy that made Harry think the Weasleys would bring doom to him, so he declared the blood feud.
Maybe in that fanfic, there are misunderstandings between Harry and the Weasleys. For example, they might have had an argument over something like magic use or family values that led Harry to dislike them.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
One possible reason could be a misunderstanding. Maybe some dark magic was involved that made Harry believe the Weasleys were acting against him or the wizarding world in a way that seemed illegal. For example, if there was a spell that distorted his perception of their actions.
One possible reason could be that Harry has post - war trauma. He might have seen so much death and destruction during the war that he doesn't want to bring his darkness and pain to the Weasleys. He may think that staying away is a way to protect them from his own emotional turmoil.
In this fanfiction, perhaps there's some miscommunication. Maybe the Weasleys' boisterous nature is misconstrued by Harry. For example, their large family dynamic might be overwhelming to him at times, which could be a reason for the 'hate' in the fanfic.
Well, it would definitely strain the relationships that Harry had with other characters related to the Weasleys. For instance, Hermione might be caught in the middle, having to choose between her loyalty to Harry and her friendship with the Weasleys. Ron, being a Weasley, would be in a very difficult position, and their friendship might be completely broken.
Perhaps Harry sues the Weasleys because he believes they misused some of his fame for their own financial gain. For example, they might have used his name to promote their joke shop without his permission. And Harry, being fed up with this exploitation, decides to sue them to set things straight.
Well, one reason could be that he was poisoned. In the world of Harry Potter, there are many poisonous substances. A fanfic writer might create a scenario where Harry is unknowingly poisoned, and this causes him to throw up blood as his body tries to fight off the toxin. Another possibility is that he has some sort of internal injury from a fight. If he got hit by a powerful spell or a physical attack during a battle, it could lead to internal bleeding which results in throwing up blood.
One reason could be jealousy. In some fanfictions, if Harry has become very famous and powerful, he might think the Weasleys have different motives for being around him, like using his fame for their own gain. So he starts to lose trust in them.
One possible reason could be jealousy. If Harry gets more attention or power than the Weasleys, they might feel envious and turn against him.