One possible reason could be a misunderstanding. Maybe he discovers some secrets about his parents that he misinterprets. For example, if he finds out they were involved in some dangerous or morally ambiguous activities in their youth that he doesn't fully understand at first.
It could also be due to outside influence. If there is a powerful dark force or a manipulative character in the fanfiction that convinces Harry that his parents are not what he thought they were. They might make him believe that his parents' actions led to some great harm that he can't overlook.
One possible reason could be that he discovers some long - hidden family secrets that make him question his parents' actions or decisions. Maybe he finds out about some dark deeds in their past that he can't overlook.
One possible reason could be that in the fanfic, the author might twist the story to make it seem like his parents' actions had unforeseen consequences that affected Harry negatively. For example, their fight against Voldemort led to Harry being an orphan and having to bear a great burden from a young age.
One possible reason could be that in the fanfiction, Harry might have discovered some dark secrets about his parents that made him feel betrayed. For example, if he found out they had some hidden and unethical magical experiments in their past.
Perhaps he discovers some secrets about his parents that he doesn't like. For example, he might find out that they were involved in some dangerous or unethical magic before. This new knowledge could make him angry at them for putting themselves and their family in jeopardy.
In some fanfictions, it might be due to an alternate reality. For example, if there was a different prophecy or a different turn of events at the time Voldemort attacked. Harry could leave his parents to fulfill a new - found destiny in this altered world. Also, it could be because of a miscommunication. A magical misunderstanding might lead Harry to believe he has to leave his parents for their own safety.
On a more magical level, reviving the dead is against the natural order of things in the wizarding world. This could attract unwanted attention from magical authorities or even dark forces. Some might see Harry's act as a dangerous precedent and try to either stop him or use his new - found power for their own evil purposes.
Well, it completely flips the typical relationship dynamic. Usually, Harry idolizes his parents. But in this fanfiction, by disowning them, it creates a sense of conflict and chaos. It might make the story darker as Harry is now sort of an outcast not just in the wizarding world in general but also from his own family legacy.
Maybe they were forced into it by some dark magic or a powerful dark wizard. They could have been blackmailed into betraying Harry to save their own lives or the lives of others they cared about.
One possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
Yes, there might be some extremely dark and out - of - canon fanfictions out there with such a disturbing premise, but it goes against the general nature of the original story where Harry's parents loved him deeply. However, in the vast world of fanfiction, people can create all kinds of strange and often inappropriate scenarios.