In this fanfiction scenario, Dumbledore could be acting out of desperation. He may have seen a future where Harry is in great danger and the normal means of protection within Hogwarts are no longer sufficient. So, he resorts to kidnapping Harry. It could also be related to some secrets about Harry's family or his own power that Dumbledore has been keeping. He might think that by taking Harry away, he can finally reveal these secrets to him in a controlled environment and train him better for the upcoming battles.
One possible motive could be protection. Dumbledore might tie Harry to the bed to keep him from running off into danger. He has always been protective of Harry and perhaps in this fanfiction's unique scenario, he believes this extreme measure is necessary to safeguard Harry from some immediate and unseen threat.
Perhaps in this fanfiction, Dumbledore has a secret plan related to Harry's power. He might think that by kidnapping Harry, he can train him in isolation to fully develop his magical abilities without the distractions and potential dangers at Hogwarts. It could also be that Dumbledore is trying to hide Harry from Voldemort in a way that no one would suspect, as Voldemort would never think Dumbledore would use such extreme measures against Harry.
Maybe Dumbledore believes that Harry has some hidden power that is being corrupted by the environment at Hogwarts. So, he kidnaps Harry to a secluded place to train him and help him control this power without any distractions or negative influences from other students or even some of the teachers.
Dumbledore might be motivated by his over - arching plan to defeat Voldemort. He could see Hermione as a sacrifice that needs to be made. Maybe he believes that by betraying her, he can gain some crucial information from the enemy that will ultimately lead to Voldemort's downfall.
Well, in some fanfictions, Harry might attack Dumbledore if he discovers that Dumbledore has been using him as a pawn in a bigger game all along. This feeling of betrayal could drive him to lash out. Also, if there is a prophecy that he misinterprets to mean Dumbledore is a threat to his future or the future of his friends, that could be a motive.
In a fanfiction, Dumbledore might steal out of desperation. Suppose there was a situation where a curse was about to be unleashed on Hogwarts, and the only way to stop it was by stealing a particular ingredient for an antidote from a place where it was not supposed to be taken. Dumbledore, seeing no other option, decides to steal it. His motive here is to save the school and its students, even if it means breaking the rules.
Well, perhaps Dumbledore has some dark secret and believes Harry is the key to it. He might kidnap Harry to keep him away from Voldemort in a very unorthodox way. Maybe he wants to train Harry in isolation for some super - powerful magic that he doesn't want Voldemort to find out about.
One possible motive could be a misguided sense of protection. Dumbledore might think that taking something from Harry, like a powerful magical item, would keep it out of the wrong hands. He may believe that Harry is too young or inexperienced to handle it properly.
Perhaps Dumbledore foresaw some sort of prophecy that was related to Harry and his parents. In order to fulfill the prophecy in the right way or to avoid a greater catastrophe, he kept Harry from his parents. He was always thinking about the greater good of the wizarding world and might have believed this was a necessary step.
One possible motive could be that Dumbledore saw a future where Harry's parents' deaths were necessary for a greater good. Maybe he knew that Harry would be the only one to defeat Voldemort in the end, and their deaths were a catalyst to set Harry on that path.