Well, it might be because some fans feel that Slytherin has been somewhat demonized in the original books. So, they create fanfiction where Slytherin is the victim of abuse, perhaps as a way to give the house more depth and sympathy. Also, the idea of abuse can create a lot of drama and conflict, which are elements that make a story interesting. Fanfiction writers might be using this concept to attract readers who are interested in more complex and edgy storylines. Additionally, it could be a form of exploring the morality and ethics within the Harry Potter universe in a very extreme way.
There are several possible reasons. Firstly, the need for creative exploration. Some fans want to break away from the typical storylines and explore uncharted territories in the Harry Potter world. Secondly, it could be a reaction to the perceived unfair treatment of Slytherin characters in the original story. By creating a scenario where Slytherin is abused, they are highlighting the potential for prejudice and the consequences of it. Thirdly, it might be an attempt to add more layers to the characters. If Harry, who is generally seen as a hero, is made to be an abuser, it completely changes the perception of his character and allows for a more in - depth analysis of his personality, as well as the nature of good and evil in the series.