It could also be a form of creative rebellion. Fans might want to explore a different side of the 'Harry Potter' universe. They think that by making Harry Potter seem 'bad' in relation to Snape, they can tell a new and unique story. They are not satisfied with the traditional view of Harry as the hero and Snape as the somewhat villainous figure, so they reverse it in their fanfiction to see where it leads.
One reason could be that the creators of such fanfiction are die - hard Snape fans. They might feel that Snape is the most interesting and underappreciated character in the 'Harry Potter' series. So, they create fanfiction where Harry Potter, who they may see as getting too much attention in the original, is not as great, to put more focus on Snape.
Perhaps the fans writing this type of fanfiction are trying to delve deeper into Snape's emotional and psychological state. They believe that Harry Potter's presence and actions in the original story prevent a full exploration of Snape. So, in order to give Snape more space and significance, they make Harry Potter look bad in their fanfiction. This allows them to build a world where Snape can be fully understood and his character can be more fully developed without Harry's interference, at least in their view.
A theme could be redemption. Perhaps Snape has done something so unforgivable in the eyes of Harry that it makes Harry think Snape sucks. And the story could be about Snape trying to redeem himself in Harry's eyes. Or it could be about jealousy. Snape may be jealous of Harry's fame and Harry may sense this and thus dislike Snape.
One possible reason is to show a contrast. Since Bill is often seen as a reliable and good - natured character in the original series, making him 'suck' in fanfiction can create a shock value and make the story more interesting. It can also be a way to explore the flaws of a seemingly perfect character.
Some authors might create it as an exercise in 'what - if' scenarios. They want to break the established narrative and see how different characters would react if Harry was killed so early. Also, it could be a way to explore the Dursleys' characters more deeply, showing just how far their hatred for Harry could go.
Perhaps in this fanfiction, Snape shows a side of himself that is completely different from what Harry has known. He might be more open and honest with Harry, sharing secrets that make Harry believe he can be trusted. Like Snape might disclose some long - held information about Harry's parents that only he knew, which makes Harry think Snape has been on his side all along.
Well, if such a fanfiction exists, one possible reason could be a huge misunderstanding. Maybe Harry discovers something about Snape that he misinterprets and then hounds Snape with accusations and guilt - tripping until Snape can't take it anymore. Another reason could be if dark magic is involved in some way, like Harry is under the influence of a dark spell that makes him act out of character and target Snape relentlessly.
One possible reason could be copyright infringement. If the fanfiction uses too much of the original 'Harry Potter' material without proper authorization, J.K. Rowling (or those in charge of the franchise) might take legal action. Another reason could be if the fanfiction misrepresents the characters in a way that goes against the intended spirit of the original work.
It could be that in the context of the fanfiction, Snape believes Harry has been disrespectful towards him or other teachers. For example, if Harry were to talk back or show disobedience in class, Snape would likely punish him as a way to assert his authority.
One reason could be that he realizes Harry has a lot of potential and wants to nurture it. He sees that Harry is in a difficult position, being the target of Voldemort, and decides to help him despite their past differences.
One possible reason could be Snape's lingering resentment towards James Potter, Harry's father. Snape might see Harry as a reminder of the bullying he endured from James during their school days. So, he takes out his anger on Harry.
Some might create it just for shock value. They want to break the norms of the well - known Harry Potter universe and get people talking. It's a way to be edgy and different from the typical fanfiction that stays closer to the original themes.