There are several motives. Firstly, to add a sense of tragedy. The 'Harry Potter' series has its share of losses, but Hermione's death in a fanfic at Malfoy Manor would be a new level of tragedy. It could also be to rewrite the story in a more 'grimdark' style. This would attract readers who are interested in seeing a different side of the wizarding world. Moreover, it might be an attempt to explore the consequences of a major character's death on the overall plot. For example, how would the fight against Voldemort be different without Hermione's intelligence and magic skills?
It's a rather unique plot twist. In most of the original 'Harry Potter' series, Hermione is a strong and central character who survives. Having her die at Malfoy Manor goes against the general narrative we expect. Fanfic authors might choose this to create a very different emotional impact and explore how the other characters would react to such a huge loss.
Perhaps they are there to rescue someone. Hermione might come up with a clever plan involving potions and charms to get past the manor's defenses. Ron, with his loyalty and determination, would be right by her side, ready to take on any danger that comes their way.
In some fanfictions, Ron and Hermione could be at Malfoy Manor for a diplomatic mission of sorts. The wizarding world is in chaos, and they are sent to try and convince the Malfoys to switch sides. This would be a very difficult task as the Malfoys have long been associated with the dark side. But Ron's loyalty and Hermione's persuasion skills could potentially make a difference.
It could also be a result of a magical bargain or curse gone wrong for the Malfoys. Suppose there was a powerful dark magic object in the manor that got out of control. Hermione, with her vast knowledge of magic, manages to contain it. In the process, she ends up being the rightful owner according to some ancient magic laws. This would make for an interesting plot twist in the fanfiction.
One possible plot could be that Hermione is captured and brought to the Malfoy Manor. Draco, having some internal conflict due to his growing feelings for Hermione despite their past enmity, tries to protect her from the other Death Eaters in the Manor. Maybe they end up having to work together to escape some dark magic trap set within the Manor.
Well, one scenario could be that they are attending a peace - making event. Hermione, being the diplomat, tries to make amends between the Weasleys and the Malfoys, and Ron is there just to support her and make sure she doesn't get into any trouble. But things quickly go awry as old rivalries resurface.
In fanfics where Hermione is Draco's mate in Malfoy Manor, it could begin with a situation where Hermione discovers a side of Draco that no one else has. Maybe he shows a bit of kindness towards her when she's in a vulnerable position at the Manor. This small act could make Hermione re - evaluate her view of him. At the same time, Draco might be intrigued by Hermione's intelligence and tenacity. Over time, these feelings grow and they become mates, often defying the social norms of the wizarding world.
One thing that makes it interesting is the contrast between their backgrounds. Hermione, a Muggle - born with strong values of equality and knowledge, and Draco, a pure - blood from a family with a long history of prejudice. When they come together as mates in the context of Malfoy Manor, it creates a lot of tension and potential for character growth. They have to overcome their initial differences and the expectations of their respective social groups.
One possible motive could be Hermione's sense of justice. She might see Snape as a misunderstood figure who has been doing things for the greater good all along, even if it doesn't seem that way on the surface. So, she decides to sacrifice herself to save him.
After Malfoy Manor, their relationship could also become more public within the wizarding community in fanfiction. They may face different reactions from friends and family. Some might be overjoyed seeing them finally together, while others might be a bit skeptical. But through all this, Ron and Hermione would grow closer, learning to deal with external pressures as a couple, and also finding new ways to support each other in the fight against the Dark Arts.