One possible motive could be fear. Voldemort is extremely powerful and if he threatened Hermione or her family, she might feel compelled to spy for him. Another motive could be a thirst for knowledge. Voldemort might have promised her access to dark magic knowledge that she couldn't get anywhere else.
In some fanfics, Hermione could be a spy for Voldemort due to a sense of inadequacy. She might feel that she's not fully accepted in the wizarding world among the good guys. Voldemort's side could have made her feel special, like she was part of an exclusive group. They might have flattered her intelligence and told her she could be a great leader in their new world order, which led her to become a spy for them.
She could have been misled. Maybe Voldemort or his followers tricked her into believing that the Death Eaters had the right ideology. They could have made false promises about a better wizarding world under their rule. Also, she might have been influenced by some dark magic object that clouded her judgment and made her side with Voldemort.
In a fanfic with Hermione as Voldemort's spy, it's a very interesting concept. First of all, Hermione is known for her intelligence and loyalty in the original series. But if she's a spy, her intelligence could be used against the good guys. She could be using her knowledge of spells and magic to deceive her friends. The plot could involve her gradually realizing the error of her ways and trying to turn against Voldemort. Or perhaps she's so deep in the spy role that she becomes a real antagonist, causing chaos and destruction among the characters we love.
One possible motive could be blackmail. Maybe the dark forces have something on her or her loved ones that forces her to spy for them. Another could be a thirst for knowledge. In the 'Harry Potter' series, Hermione is always eager for knowledge. If she thought the dark side had some unique knowledge or power she couldn't get elsewhere, she might be tempted to side with them, at least as a spy.
One motive could be that Voldemort wants Harry on his side. He might think that by rescuing Harry from Vernon, who was clearly a bad caretaker, Harry would be more inclined to listen to him and join him.
One possible motive could be for Harry's protection. Dumbledore might have seen a threat that Harry wasn't aware of, and he asked Hermione to keep an eye on him. Hermione, being the loyal friend and also having great respect for Dumbledore, would have agreed to it.
Another motive might be related to a prophecy or some ancient magic. There could be a prophecy about Harry that certain people want to manipulate or prevent from coming true. By paying Hermione to spy, they hope to get insights into Harry's actions and decisions that might be related to this prophecy.
Maybe she could be motivated by power. The Dark Lord might have promised her a high position in his new world order. She might also be disillusioned with the wizarding world's current state and think that the Dark Lord could bring about a better change, even if it means using dark magic.
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?
One possible motive could be a deep - seated dissatisfaction with the wizarding world's status quo. Maybe she felt that the Ministry of Magic was corrupt and that the Death Eaters had a better vision for change, although a very dark one. Another could be some sort of personal vendetta against someone in the Order of the Phoenix or Hogwarts faculty that made her side with the Death Eaters.
In some fanfictions, Snape might be working with Voldemort because he has been blackmailed. Voldemort could have found out some deep, dark secret about Snape and is forcing him to cooperate. This could be related to Snape's past or something he has done that he doesn't want to be made public.
One way it could be developed is through a powerful dark magic influence on Hermione. Maybe Voldemort discovers a way to manipulate her mind using a long - forgotten spell. This could cause her to gradually turn against her friends and side with him. For example, she might start to believe in Voldemort's ideas of pure - blood supremacy after her mind is clouded.