Often, Dumbledore as Death would be a very complex character. He may not be the straightforward evil or good. His role might be more about maintaining a balance in the wizarding world, which is similar to the concept of Death in some mythologies. He could be using his influence on the different characters in different ways. For instance, his relationship with Snape could be a part of his plan as Death. Snape's actions, which were so crucial in the original story, might be reimagined as being directly or indirectly guided by Dumbledore - as - Death. This could add a whole new layer of mystery and intrigue to the fanfiction.
One key plot point could be Dumbledore's true identity being revealed gradually. Maybe Harry starts noticing strange things about Dumbledore that make him suspect he is not just an ordinary wizard but Death itself. Another could be how the prophecy about Harry and Voldemort fits into this new paradigm where Dumbledore is Death. It might change the meaning of the prophecy completely, as Dumbledore - as - Death could have a different intention behind the events related to the prophecy.
Dumbledore, as Harry's guardian in the fanfiction, would probably use his influence in the wizarding world. He could make sure that Harry is taught by the best teachers at Hogwarts. He'd also be there to offer advice and guidance, especially when Harry faces challenges like dealing with Voldemort or other dark forces. For example, he might teach Harry some advanced defensive magic privately to better protect himself.
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.
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.
It's a really horrible idea. In the original 'Harry Potter' series, Dumbledore is like a father figure to Harry. Their relationship is based on trust, guidance and mutual respect. This idea completely subverts the positive image and relationship established in the original work.
I haven't come across such a disturbing fanfiction myself. In the official 'Harry Potter' lore, Harry and Dumbledore have a mentor - mentee relationship and there's no such violent scenario. Fanfiction that goes against the core values of the original work like this is not only untrue to the spirit but also quite abnormal.
He becomes more nurturing. Dumbledore, who was mainly seen as a wise leader, now has to be a caregiver. He might show more patience and tenderness as he takes on the role of a father to Harry.
Another possible antagonist could be a group within the wizarding world who doesn't want Dumbledore's plans to be exposed. They might have been benefiting from Dumbledore's secrecy or they believe that his plans, although seemingly unethical to Harry, were necessary for the greater good. They would then try to stop Harry from revealing the truth.
I recommend "The Journey of Dellusion from the Demon's Tail" to you. Part of it is about the plot of transmigrating into the Dark Lord in the diary. The main character of the delusional man, Munt, had a hobby of writing a diary. He was on a cross-world trip, which included the world of comics. He even sold a part of his diary to Dumbledore. If you like this kind of time-travel and comics style, this book will make you like it. I hope you like this fairy's recommendation. Muah ~😗