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.
She would probably be very conflicted. Hermione is Harry's friend, and spying on him goes against her loyalty. So she might be constantly struggling with her conscience.
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 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.
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.
Another motive could be related to her family. Maybe there was some sort of blood feud or an old grudge against the Grangers. The kidnappers could have thought that by taking Hermione, they could get revenge on her family. In some fanfictions, there are secret magical families that have long - standing rivalries, and Hermione's kidnapping could be part of that.
One possible plot could be that Ron and Hermione suspect Harry is involved in some dangerous secret magic that could put him in harm's way. So they spy on him to find out more and end up discovering it's all a big misunderstanding. Maybe Harry was just trying to surprise them with a new spell he learned.
Another motive might be related to a dark object influencing her. If Hermione came across a cursed object, like a dark locket that amplified negative emotions such as envy or fear, it could drive her to kill Harry. Maybe she started to envy Harry's fame and the special attention he always got, and the locket pushed her over the edge.
It could be the family butler. They often have access to all parts of the house and family affairs, and can easily eavesdrop on conversations.
Voldemort's remaining followers could be her enemies. They might be planning something new and Hermione, as a spy, would be trying to stop them. Also, some corrupt Ministry of Magic officials who are up to no good. They would see Hermione as a threat to their secret operations.
One possible motive could be money. If someone offers a large sum to spy on friends, some might be tempted. For example, in a business context, spying on friends' startup ideas could be lucrative.