There are many 'Sirius Hurts Snape' fanfictions because their relationship in the original 'Harry Potter' series is so complex. Sirius and Snape have a long - standing feud. Sirius, with his brash and sometimes cruel behavior towards Snape, gives a lot of room for writers to explore the consequences of his actions on Snape. It also allows for exploration of Snape's more vulnerable side which is not always shown in the books.
There's a fanfiction called 'Lingering Pain' that's part of the 'Sirius Hurts Snape' genre. This one focuses on the psychological damage Sirius inflicts on Snape over time. The author does a great job of showing Snape's struggle to cope with the constant jabs from Sirius. Additionally, some of the more niche fanfictions like 'The Wounded Soul' explore the idea of how Sirius's actions towards Snape are not just one - off incidents, but part of a pattern that deeply scars Snape and shapes their relationship in a very negative way.
Snape might be secretly pleased at first, seeing as he has no love for Sirius. But then, being a teacher, he'd probably feel obliged to report it. He also might use this situation to his advantage against Sirius in the wizarding politics within Hogwarts.
Sometimes, Sirius's own secrets can harm Harry. In some fanfictions, if Sirius hides important information about Harry's family or his own past, and Harry discovers it later in a bad way, it causes emotional pain. A good example is 'The Hidden Truths' where Harry finds out Sirius knew something crucial all along.
I haven't come across a 'snape is sirius slave' fanfiction myself. Fanfiction themes can be very diverse, but this one seems rather unorthodox and might not be as common. However, the world of fanfiction is vast, and new works are being created all the time, so it's possible that someone has written it, but it may be difficult to find.
Yes, there are some. You can start by looking on fanfiction platforms like Archive of Our Own. Just search for 'Snape hurts Hermione' in the search bar and you'll likely find a variety of stories. However, keep in mind that the nature of this concept can be a bit dark, so be prepared for some intense and perhaps disturbing content.
One common theme is betrayal. Sirius might do something that goes against James's trust, like revealing a secret or making a decision that impacts James negatively without his consent.
Sirius could face serious trouble. Snape would surely report him to Dumbledore. Dumbledore might have to take action like restricting Sirius' access to Hogwarts or even sending him back to Azkaban if the harm to Harry was severe enough.
You could also follow some fanfiction writers who are known for writing about the Harry Potter universe. Check their works as they might have written something related to Sirius hurting Remus.