It could be due to a misinterpretation of his relationship with his students. Snape was strict and often seemed cold, but that doesn't mean he had any inappropriate feelings. Some fanfiction writers may not fully understand his character's depth and complexity and so create these false portrayals. Also, in some cases, it might be the result of a writer with a warped sense of creativity who wants to deviate drastically from the original character for their own strange narrative purposes.
Well, sometimes in the world of fanfiction, there are those who like to take well - known characters and distort them in the most extreme ways. For Snape, it could be that some writers don't appreciate his real motives and relationships in the original story. They see his interactions with students like Harry and misread them completely. Instead of seeing his tough love or his role in the bigger plot, they jump to this completely unfounded and immoral conclusion just to make a 'unique' story, without considering the damage it does to the character's original integrity.
Some fanfictions might do this out of a desire for shock value. They want to create a controversial and extreme version of the character to get more attention.
Some writers might do it for the novelty. It's different from the traditional Hermione we know. It allows for a fresh take on her character and can lead to really unique storylines.
Some fanfictions portray Naruto as quiet because it's an interesting character deviation. It allows the author to explore different aspects of his personality. Maybe they want to show a more introspective side of Naruto that wasn't as prominent in the original series.
Often, fanfictions show a very strained relationship between them. Given Tobias's likely abuse of Severus in the original story, many fanfictions expand on Severus's internal struggle with his father. For instance, Severus might be constantly haunted by memories of his father's drunken rages.
Some fanfictions portray Harry as mean to Draco because it adds a new dimension to their relationship. It breaks away from the typical hero - villain dynamic and shows that even the 'good' character can have flaws. It also allows for more complex character development. For example, Harry might be mean due to jealousy over Draco's family wealth or status at Hogwarts.
Well, it could be that the authors want to explore Snape in a different setting. Britain has so many ties to his past, especially his time at Hogwarts and his role in the war. By leaving Britain, fanfiction writers can show how he copes without those familiar surroundings. He can be a different person, or maybe find a new purpose in life. It also allows for new character interactions as he meets people outside of the British wizarding world.
Some authors write these fanfictions to show different sides of the characters. For example, Snape's action of hitting Harry could be a result of his own inner turmoil, perhaps related to his loyalty to Dumbledore and his hatred for James Potter. By writing about this, they can show Snape in a new light. Also, it can show Harry's growth in dealing with such a difficult situation. He can either become bitter or try to understand Snape's motives, which are interesting character development arcs.
They often show him as a deeply conflicted and damaged individual. His past experiences, like his difficult childhood and the complex relationships in his life, are used to explain his tortured state. For example, his unrequited love for Lily might be explored in more depth, and how that has tortured him over the years.
Some fanfictions show Harry and Snape having very tense interactions. Even though the war is over, the years of hatred and misunderstanding don't just disappear overnight. Snape might still be sharp - tongued with Harry, and Harry might be on edge around Snape.
Often as a complex figure. He's still the reserved and somewhat prickly man we know, but in the context of marriage, his softer side might start to show. For example, he may be more protective of his partner.
Some people like it because it's a pairing that breaks the norm. In the original 'Harry Potter' series, their relationship is mainly that of a strict teacher and a student. Fanfictions allow for a different kind of exploration, a 'what - if' scenario where they could be more than that.