It depends on how the author writes it. If it's very graphic and violent, it might not be suitable for younger readers. But some authors can handle the dark theme in a more tasteful and less disturbing way.
One possible motive could be jealousy. Bellatrix is extremely loyal to Voldemort, and Hermione is a very powerful and intelligent witch from the side opposing Voldemort. Bellatrix might see Hermione as a threat and also be jealous of the attention Hermione gets from Harry and Ron. So, she tortures Hermione to try and break her spirit and prove her own superiority.
Their relationship could start with hostility. Bellatrix, as a former Death Eater, would initially despise Hermione for being a Muggle - born. But as the story progresses, they might find common ground in their love for magic. For example, Hermione's vast knowledge of magic could intrigue Bellatrix, and she might start to see Hermione as more than just a 'Muggle - born scum'.
She might be more conflicted. Knowing her mother is Bellatrix, she'd struggle with her own moral compass. Maybe she'd have a darker side to her magic that she has to constantly fight against.
Well, Bellatrix is a very complex and cruel character. In the fanfiction context, she might torture Draco to assert her dominance. She is a powerful and deranged Death Eater, and by torturing Draco, she can show that she is in control. Also, it could be related to her pure - blood mania. If she thinks Draco is not living up to the pure - blood standards in some way, she may take it upon herself to 'correct' him through torture.
Hermione's resilience can also be shown by her not breaking down easily. She might endure the pain and still taunt Bellatrix, showing that she is not afraid. This could be portrayed by her making sarcastic remarks despite the torture. Another way is that after the torture, she quickly recovers and is more determined than ever to fight back.
Hermione might resist by trying to escape. She could look for weaknesses in the place where she is being tortured. If there are any magical objects or elements around, she might try to use them to her advantage. Also, she could try to form an emotional bond with Bellatrix in a strange way. By appealing to Bellatrix's possible humanity or some shared experience within the wizarding world, she hopes to make Bellatrix stop the torture. However, this is a very risky move as Bellatrix is known for her cruelty.
One plot idea could be that Bellatrix, in an alternate reality, has a change of heart and decides to protect Hermione from other dark forces. Maybe she sees a bit of herself in Hermione's fierce determination. Another could be that Hermione, while researching dark magic for a school project, accidentally stumbles upon a long - forgotten connection between her family and Bellatrix's, leading to a strange alliance. And a third idea might be that Bellatrix is cursed and Hermione is the only one who can break the curse, but in the process, they have to face their deep - seated prejudices about each other.
Maybe introduce a deus ex machina element. For instance, just when it seems the torture is getting too much, a powerful magical creature appears and scares Bellatrix away. This would lighten the mood a bit and prevent the story from going into extremely dark territory. Also, the author could play with the idea of Hermione having some sort of pre - prepared magical protection that lessens the impact of the torture.