Some people write anti - Snape fanfiction because they see him as a bully. He was often mean to students, especially Harry, and they feel the need to explore the negative aspects of his character more deeply in their writing.
Another reason could be that they want to explore alternative storylines. If Snape was not the character he was in the original, how would it affect the whole Harry Potter universe? By writing anti - Snape fanfiction, they can change his role and see how the relationships in Hogwarts, the battles against Voldemort, and the overall plot would be different. For example, they might write a story where Snape's actions lead to a more disastrous outcome for the wizarding world.
People might write anti - Snape fanfiction because they don't believe his redemption arc was well - deserved. His past actions of being a Death Eater and his general nastiness towards the students don't seem to be fully compensated by his later actions. So they write to re - evaluate his character without the positive light the original story gave him in the end. He was also rather vindictive, like when he took out his grudges on Neville, which could be a strong motivation for writers to create anti - Snape stories.
Some people write anti Naruto fanfiction because they want to explore different perspectives. They might not agree with the original story's direction or the character development of Naruto. So they create their own versions to satisfy their creativity.
Well, it's also about exploring new relationships within the Harry Potter universe. Snape has his complex backstory and Luna has her unique charm. Writers like to imagine how these two could interact outside of the main storylines. It might be a way to give both characters new dimensions and story arcs that the original books didn't fully explore. For example, Snape could find some sort of peace or redemption through his relationship with Luna, and Luna could gain a more in - depth understanding of the darker aspects of the wizarding world.
Some people write anti - Sakura Haruno fanfiction because they don't like her character development in the original series. They might feel that she didn't grow in a way that was satisfying to them.
Some people write it to explore different aspects of the Naruto characters. Maybe they want to see how a character would develop if they were not bound by the social rules of the Naruto world.
Some people write it because they feel the relationship between Sasuke and Sakura was forced in the original series. They might think that Sakura could have had a better romantic option or that Sasuke was never really that into her.
There could be a sense of wanting to rewrite the narrative. Maybe they felt that Snape was over - glorified in the original story, especially considering his past as a Death Eater. They want to explore the idea that his actions, even his good ones like protecting Harry, were not enough to make up for his earlier wrongdoings. So they write fanfiction to challenge the positive view of Snape that some fans have.
Another reason could be that some writers like to explore different storylines and universes. They might not be satisfied with the canon story's romantic plot and want to create their own versions. In the case of 'anti naruhina fanfic', they can rewrite the story without the Naruhina relationship and see how the story unfolds differently. This gives them creative freedom to play with the characters and the Naruto universe.
People may write it as a form of creative rebellion. The Pokemon franchise is so popular and has a very set image. By creating 'anti pokemon fanfic', they can break the mold and create something that stands out. It could also be a way for them to express their own views on things like animal rights (comparing Pokemon to animals in a sense) if they think the way Pokemon are treated in the franchise is not ideal.
Some people write anti mpreg fanfic because they prefer to stick to the traditional gender roles and biological norms represented in the original source. They might feel that mpreg is too much of a deviation from what they know and love about the characters and the story world.
Some people write 'anti Bakugou fanfic' because they don't like his aggressive and bullying nature in the original story. They want to explore scenarios where he is held accountable for his actions more than in the official plot.