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.
In some fanfictions, the motive might be a sense of disillusionment. If Hermione feels let down by the good side, she could potentially be swayed to the dark side as a spy. For example, if she thought the good side was being hypocritical or ineffective in fighting the real threats. Also, she could be under the influence of a powerful dark magic spell that made her act as a spy without her full conscious control.
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.
It could be a revenge plot gone too far. Suppose her parents did something in the past that she just found out about, and in her new dark persona, she decides to take the ultimate revenge. Or perhaps she has fallen so deep into the dark side that she no longer values family and sees them as just another obstacle in her path of darkness.
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.
One possible motive could be jealousy. Hermione is highly intelligent and often outshines Draco in academics. In a twisted version of a rivalry, Draco's envy could turn into a deadly hatred. Another could be his family's influence. If the Malfoys push him to take extreme actions against Muggle - borns, Hermione would be a prime target.
Well, it depends on the specific fanfiction. There are countless fanfictions out there with all kinds of wild ideas. Some authors might create a scenario where Hermione is a spy for the dark just to add an unexpected twist to the story. But in the traditional sense, it goes against what we know of her as a smart, brave and good - hearted character.
Jealousy might be a motive. If Hermione is constantly outshining them in academics or getting more praise from teachers, they could become envious and turn against her.
One possible motive could be Bellatrix's loyalty to Voldemort. She might think that by kidnapping Hermione, she can get information about Harry Potter and the Order of the Phoenix. Hermione is known to be very intelligent and Bellatrix could believe she has valuable knowledge. For example, Hermione might know the location of Potter or the Order's plans against Voldemort.
Maybe Rowle is under the orders of a more powerful dark wizard. In the world of Harry Potter, there are always those higher - ups in the dark side who might want Hermione for her knowledge of magic, especially things like ancient spells and magic theory. So Rowle could be tasked with kidnapping her for that purpose.
One possible motive could be to use Hermione as a bargaining chip against Harry Potter. Since Hermione is his close friend, the kidnappers might think they can force Harry to do something, like give up a powerful magical item or valuable information.
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.