There are some fanfictions like this out there. One possible reason for such a plot could be exploring the 'what - ifs' in a very extreme way. Maybe the author wants to show how far a character can be pushed from their normal state. It could also be a way to explore the consequences of a broken mind or a character being corrupted by external forces. But it's a really sad and hard - to - accept concept considering the love and bond between Harry and Sirius in the original story.
In this type of fanfiction, the author may be trying to create a shock factor. Harry killing Sirius could be set in a parallel universe where everything is distorted. For example, Voldemort could have won earlier and manipulated Harry's mind to turn against his loved ones. Or there could be a new, extremely powerful dark magic artifact that makes Harry lose his sense of self and attack Sirius. But it's important to note that this is a very different take from the original, positive and hopeful tone of the Harry Potter series.
Yes, there might be some fanfiction with such a dark and alternate storyline. However, this goes completely against the essence of their relationship in the original series. In the canon, Sirius is a very important figure in Harry's life, like a father figure. So in this kind of fanfiction, it could be part of a dark magic - influenced scenario, perhaps Harry being under the Imperius Curse and forced to do it, or some extremely warped perception of reality that makes him think Sirius is a threat.
In such a fanfiction, it could be that Harry Potter has been corrupted by a powerful and malevolent force. This force might have warped his sense of right and wrong. It could also be that the author wants to explore the extreme consequences of power and how it can turn a hero into a villain. For example, if Harry Potter was constantly taunted and mistreated by the wizarding community despite all he has done, he might reach a breaking point and lash out at everyone.
One possible motive could be misinformation. If Harry is fed false information about Sirius being a traitor or a danger, he might act on it. For instance, if a well - disguised Death Eater manages to convince Harry that Sirius has joined Voldemort, Harry, in his haste to protect others, could attack Sirius.
The wizarding world would likely be in a state of shock and then celebration. People would start to see Azkaban in a new light, perhaps no longer as just a place of horror but also as the site where the great evil was finally vanquished.
He might gain more respect from his peers. After all, killing the Basilisk is no small feat. His friends would probably look up to him even more, and his reputation in Hogwarts could soar. He could also become a bit of a legend among the younger students.
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.
In a more complex scenario, Bellatrix could be trying to use a dark magic ritual that would cause great harm to the wizarding world. Harry discovers her plan and has to engage her in a magical struggle. In the process, he manages to turn her own dark magic against her, which results in her death. This would show Harry's ingenuity and his ability to use the enemy's own weapons against them.
Well, it could be a story where Harry is tricked into believing that Muggles are responsible for some great harm to his loved ones in the wizarding world. So, in his rage and misdirected sense of justice, he starts targeting Muggles. But this is a very un - Harry - like behavior as the real Harry is all about protecting the innocent, whether they are wizards or Muggles.
Well, it could be that he has lost all hope. In the fanfiction world, if the situation becomes so dire that he sees no way out for himself or the people he loves. For example, if Voldemort manages to gain complete control and Harry Potter thinks his sacrifice by taking his own life might be the ultimate solution to break Voldemort's power. He might also be suffering from post - traumatic stress after all the battles and losses, leading him to this extreme decision.
Well, perhaps Pettigrew gets tired of living in fear under Voldemort. He might find a powerful magical artifact that has the potential to kill Voldemort. As he tries to steal it for himself, Voldemort discovers his plan. In the ensuing struggle, Pettigrew manages to turn the artifact against Voldemort and kills him. This would then lead to Pettigrew having to deal with the consequences, like facing the wrath of the Death Eaters who are still loyal to Voldemort.
In this fanfiction, perhaps the Nundu is causing chaos. Harry, being the hero, steps up and kills it to restore peace.