Step 2: The Exposition
Write, or more preferably just Copy a 6 paragraph long summary of the last 4 books in the series from some or the other Literature website and in thorough explain why every single bad thing that happened was Dumbledore's fault.
That's some riveting content right there. Your readers will definitely not be bored or annoyed at all.
Definitely
Next, don't forget to throw in a paragraph about how Dumbledore was porking Quirrel in the bum in the first year while paying off Ron with one hand and furiously dabbing with the other. Then comes the most important part of his setup as your story's villian. Letting your readers know that Dumbledore was also aware of everything related to the Chamber.
Now. Some readers are pesky little cunts. They try to ask for 'logic' and 'explanations'. Don't worry too much about that. Throw in whatever explanation you want to.
For example, it could be due to him being on some sort of God-cocaine shit that allows him to be omnipotent and control Dementors, because Dementors get real confused around stoned people. Using that control he sabotaged Sirius's attempt at proving himself to be innocent. He also knew from the start about Barty, and used Imperius on him to make sure that Harry was chosen in the Tournament and was almost killed by Voldemort.
Don't worry about why he'd do that. He's high on God-cocaine after all.
This step is pretty common isn't it? How many stories have you read that have this step?