One possible motive could be that Hermione believes the grimoire contains powerful magic that she thinks is necessary to defeat a great evil. She might think Harry is not using it to its full potential or is unaware of its true importance.
At first, Harry would be angry and confused. But as the story progresses, if Hermione has a valid reason for stealing the grimoire, they could start to repair their relationship. However, there would always be a shadow of doubt and a sense of unease between them. For example, Harry might be hesitant to share important things with Hermione in the future.
Yes, there are some out there. Some fanfictions explore this alternative relationship dynamic in interesting ways. They might rewrite the story's events and character interactions to make it seem more plausible for Harry to be with Hermione instead of Ron.
One possible reason could be that she is in a desperate situation. Maybe she needs something for a very important spell or potion to save someone's life and she thinks Harry won't notice or will forgive her. For example, if there is a rare ingredient that only Harry has access to and time is running out.
Maybe she's desperate for some knowledge that she thinks Harry has in those books. For example, if the books are related to some dark magic or ancient spells that she wants to study for the greater good, she might be tempted to take them without asking.
If Hermione realizes her mistake and tries to make amends, it could lead to a deeper understanding between them. They might have to work through their feelings and have some difficult conversations. For instance, Hermione might have to explain her reasons fully, and Harry would have to decide if he can forgive her.
The plot could start with Harry realizing he has deeper feelings for Hermione than just friendship. He might start spending more time with her, which causes tension between him and Ron. Eventually, he could confess his feelings to Hermione, and she might be conflicted because of her relationship with Ron.
Harry might get really angry at Hermione. This could lead to a rift in their friendship, causing them to avoid each other for a while. Their other friends like Ron would then be caught in the middle, trying to mend the relationship.
The consequences can be quite complex. Firstly, Harry might report her to the relevant authorities in the fanfiction world, like the magical law enforcement. This could lead to her facing legal penalties. Emotionally, Harry could be deeply hurt and become more guarded in future relationships. His friends might also side with him, leaving Molly isolated. Moreover, it could change the power dynamics in the story, as Harry might gain more sympathy from others, while Molly's reputation takes a huge hit.