One reason might be for the sake of creating conflict in the fanfiction world. By having a story where Sirius Black 'sucks', it can add a whole new layer of drama. For example, if the fic is about Harry having to deal with a not - so - great Sirius Black, it can explore different aspects of Harry's character as he copes with this different version of his godfather. It could also be a way for the author to stand out, as most fanfictions tend to have a more positive view of Sirius.
Well, it could be that the author has a very different interpretation of Sirius Black's character. Maybe they focused on some of his flaws or actions that they didn't like and decided to write a fic based on that. Or perhaps they wanted to create a unique storyline where Sirius Black is not the 'hero' he is often portrayed as in the original Harry Potter series.
Some people might create such fanfiction because they have a different view of the original 'Harry Potter' series. Maybe they don't like the characters, the plot, or the overall world - building. They want to express their dissatisfaction through creating their own stories that go against the general positive perception of 'Harry Potter'.
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.
A possible theme is misjudgment. The fanfiction might explore how Harry misjudged Sirius Black as a great and reliable figure, only to find out that he has some major flaws. Also, the theme of broken trust can be there. If Sirius is not as good as he seems, it can lead to a breakdown of the trust Harry has in him, which can then be a central part of the story's plot and emotional turmoil.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
In some fanfictions, it could be due to dark magic influence. A spell or a dark force might have corrupted Sirius's mind, making him turn against Harry.
One reason could be the strong bond between them in the original story. Fans might have felt that it could be developed into a father - son relationship in fanfic.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
One possible reason could be differences in their approach to fighting Voldemort. Sirius might be more reckless and Harry, having faced more direct threats, could be more cautious. Another reason could be Sirius' over - protectiveness of Harry. He might try to stop Harry from taking certain risks, which Harry sees as necessary for his growth and the defeat of the Dark Lord.
Well, in such a fanfiction, it could be due to a combination of factors. His isolation in Grimmauld Place might have taken a huge toll on his mental state. He might have felt like a burden to his friends, especially Harry. And the lack of any real future prospects, given his fugitive status and the ongoing battle against Voldemort, could have pushed him over the edge into taking his own life.
Sirius Black may hate Harry Potter fanfiction for several reasons. For one, he values his own identity and the events that actually happened in his life. Fanfiction can create alternate versions of him that he may find offensive or inaccurate. He also had a strong sense of loyalty and friendship, especially towards Harry. If fanfiction portrays these relationships in a wrong or overly dramatic way, it could be off - putting to him. Moreover, he was part of a real struggle against the dark forces in the wizarding world. Fanfiction might simplify or romanticize this struggle in a way that he doesn't respect, as he lived through the real and often terrifying experiences of that fight.