Perhaps Dumbledore is being forced in this fanfiction. There could be some dark magic or powerful dark wizard threatening Hogwarts or the wizarding world. Dumbledore, in a desperate attempt to save everyone else, gives Harry to Voldemort as a sacrifice. But Harry, being the hero he is, could find a way to turn the situation around and defeat Voldemort from within his lair.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
Snape's loyalty to Dumbledore would be a major driving force. He might see fighting Voldemort as fulfilling Dumbledore's final wishes. Snape would use the guidance and secrets Dumbledore shared with him to strategize against Voldemort. For example, if Dumbledore had some knowledge about Voldemort's horcruxes that he passed on to Snape, Snape could use that information to target them during the fight.
Well, it could be that Dumbledore has some sort of hidden device or spell that he activates when kidnapped. For example, he could have a charmed object that sends a signal to his allies, like the Order of the Phoenix. His allies then come to his rescue and he manages to escape during the chaos. Also, Dumbledore is extremely intelligent. He might outwit Grindelwald by engaging him in a complex conversation full of riddles and misdirections, which gives him an opportunity to break free.
Fred and George Weasley are very likely candidates. They are known for their love of pranks and their mischievous nature. They might see pranking Dumbledore as the ultimate challenge.
In some fanfictions, Voldemort wins through a change in the prophecy's interpretation. If the prophecy was misread, it could give him an edge he didn't originally have.
The Ministry of Magic could be responsible. They might be jealous of Dumbledore's power and popularity. They could see him as a threat to their own authority and come up with some false charges to arrest him. For example, they could claim he was hiding information about a dangerous dark wizard.
Snape can be a prominent character. Given his complex relationship with Dumbledore and Harry, his perspective could add a lot to the story. He might have some insights into Dumbledore's actions that Harry doesn't know about, and his reactions to Harry's blaming could vary from being scornful to having a moment of understanding. Luna Lovegood could also be a part of it. Her unique view of the world might make her offer some different viewpoints on the situation between Harry and Dumbledore.
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, obviously Harry Potter. He's the main protagonist in the original series and in the bashing fanfictions, he might be the one to finally see through Dumbledore's facade. Ron Weasley could also be popular. He's Harry's best friend, and in these stories, he might support Harry in his doubts about Dumbledore. And then there's Voldemort. Some fanfictions might use Voldemort as a tool to show how Dumbledore's actions or inactions affected the battle against him. For example, if Dumbledore was so secretive, it could have made it easier for Voldemort to gain power.