Well, perhaps the arrival of Harry Potter in their lives could be a major factor. His presence might disrupt their normal, mundane existence. They are forced to take care of a wizard child while they are Muggles who seem to despise anything out of the ordinary. His magical outbursts and the constant reminders of the wizarding world that come with him could be making their lives a living hell.
One possible plotline could be how the Dursleys face magical mishaps in their daily lives that drive them crazy. For example, maybe Dudley accidentally gets turned into a pig by a stray spell and the Dursleys have to deal with that chaos. Another could be that they are constantly haunted by strange magical creatures that are attracted to Harry's presence in their house.
In this fanfic, the Dursleys may be depicted as being in over their heads. Vernon Dursley, for example, might be shown as constantly angry and trying to assert his authority in a situation where he has none. Petunia Dursley could be portrayed as being torn between her dislike for magic and her sense of family obligation towards Harry. Dudley might be shown as being bullied or affected in the Muggle world because of Harry's presence in their family.
It might show their fear in a more extreme way. Instead of just being dismissive of magic, they could be absolutely terrified of it. For example, every time Harry does something even slightly magical, they could cower in a corner.
There might be fanfics that focus on the backstories of minor characters. In 'hell harry potter fanfic', perhaps there are stories about the life of a house - elf we barely knew in the original books, like Dobby's early years at the Malfoy Manor.
The plot might revolve around the 'Princess of Hell' trying to bring some sort of hellish influence into Hogwarts. For example, she could be on a mission to corrupt the students or steal some powerful magical artifacts from the school for her own dark purposes. However, Harry Potter and his friends discover her plans and try to stop her. They might face many challenges as her powers could be quite different from the usual dark wizards they've faced.
One possible plot could be that Harry, after defeating Voldemort, discovers a new and darker threat emerging from the underworld. He is somehow chosen or forced to become the King of Hell to maintain the balance between the magical world and the nether regions. He then has to learn the rules and powers of the underworld while still dealing with his friends and the remaining prejudices in the wizarding world.
In this fanfiction, perhaps the 'Prince of Hell' is an ancient evil that has been slumbering for ages. Harry, being the hero, stumbles upon some old prophecies or artifacts that lead him to this discovery. His friends may not believe him at first, but as strange events start to occur around Hogwarts, they realize that Harry might be onto something really big and dangerous. He then has to find a way to stop this 'Prince of Hell' from causing chaos in the wizarding world and the Muggle world alike.
Since I haven't read this specific fanfic, it's hard to say. But it could feature the main trio - Harry, Hermione, and Ron. They often find themselves in strange and dangerous situations, and this 'right side of hell' concept could be another one of their adventures. Maybe they are joined by other characters like Neville, Luna, or some of the Weasley family members.
I'm not sure specifically as there could be many different interpretations in a Harry Potter fanfic with such a title. It might be about a place that is on the 'right side of hell', perhaps a unique magical location in the Harry Potter universe that has some connection to the concept of hell, like a liminal space where good and evil forces interact in an unexpected way.
Well, 'oh hell no' is an expression of strong disapproval or refusal. In the context of 'Harry Potter fanfic', it might mean that someone really doesn't like a particular Harry Potter fan - created fictional work. Maybe they find the plot too outlandish, or it goes against their idea of the original Harry Potter universe.