4Dumbledore is not evil here. Maybe later, haven't decided yet, but not in the first chapter.
It all started with a half-remembered complaint one of Vernon's guests had made. Apparently his, rather wealthy, great aunt had died, and they couldn't read the will until all the living beneficiaries, some of whom were out of town, were rounded up. In the meantime, any relative was able to enter her house. by the time the will was read, several small but expensive items, including a rather nice vase the man had had an eye on, had vanished. The memory had hit him that night after Dumbledore had 'confirmed' that Kreacher had been passed to him. But what if he hadn't? What if it was just because he was a (distant) relative who'd gotten there before anyone else, like with that man's vase?
He'd immediately gone to Mrs. McGonagall. Apparently, any relative could have the will read at any time. Since it wasn't that late (only 9 pm) and most places in Diagon didn't close until 3 am she'd decided to do it that night instead of having him miss class or wait until the weekend. They'd met at the lawyer's within the hour, surprising him, since for some reason he'd expected it to be at Gringotts. There were two goblins there, in addition to the two lawyers and notary agent. There he'd learned that the will had already been read-by one Albus Dumbledore.
He'd seen red and just about destroyed the office. How dare that man keep this from him! He'd placed him at the Dursleys, kept the prophecy from him, refused to let him contact his friends, and gotten Sirius killed. AND NOW HE WAS KEEPING THE WILL SECRET! But the reason for that was revealed over the course of the next three hours.
Sirius wasn't as fine as he'd seemed, even as bad as that was. Remus had arrived an hour into the insane ramblings that Sirius had recorded. The man had gone from talking about penguins in elephant suits to his motorcycle, and back to a banana. And that wasn't explaining a prank or the objects- Sirius truly believed that these things were in the room interacting with him. A quarter of the time he was talking to them, and the rest of the time was split between staring blankly into space ( a full hour, broken randomly throughout the recording), crying, giggling hysterically, and talking to James. Harry's Godfather had said three total sentences even remotely related to the actual business he was there for: "Little James are you watching?" "Remus needs to be walked every day Molly, every day" and "I'm giving it to Harry". The last was the sentence that allowed harry to inherit everything Sirius owned, as he hadn't specified what 'it' was, and the wizengamot had ruled that, in these kinds of circumstances, 'it' meant everything, back in 1842.
Harry had been stunned to learn that there was precedent for interpreting a wizard's mad rambling in inheritance law, and had learned the difference between the wizarding and muggle worlds' versions of 'sound mind and body' from Remus as he was lead back to Hogwarts. In the wizarding world, sound mind meant not under the influence of an spell or substance, up to and including alcohol or aspirin, and sound body meant able to do magic. A squib's estate went to their nearest magical relative, even if they had to go back ten generations to find someone, and to their children if there was no living (magical) relative. When Bill learned that Sirius did not have a will, he set him up with all the materials needed, not knowing that Harry was unaware of the man's actual mental state.
The truth was, Sirius had been destroyed by Azkaban. The only reason that he'd had the mental will to escape was that the minister, wanting to gloat, had fed the prisoner a sanity-restoring potion developed specifically for dementor exposure. Unfortunately, the potion was short-term, only lasting a few hours. Sirius had stolen the guards' entire stock before he left, since they tasted like chocolate (a yummy potion!) and had gotten refills from a kind, elderly potion mistress with bad eyesight who'd taken it upon herself to owl him a vial every morning. After the events of third year, Albus had tracked her down and thanked her, before forcing Severus to brew the complicated and costly potion, which did not improve the git's attitude. When Sirius was not under the potion's influence he was a gibbering mess, but the potion itself was poisonous. If Sirius had continued to take it, he would have died in under five years. This was why the Weasley's had been living in Grimmauld, and why Molly had been so rude to Sirius, even though she was a 'guest'.
Harry had drifted through the next year in a daze. Sirius had died for him. Sirius was insane. Sirius was gone. That was all he could think about. When a strange potions book popped up, he handed it over to Hermione. He barely noticed Draco. And Dumbledore...
He wasn't going to 'get over it'. Sirius loved him, had tried to take care of him, even though he should have been the one taken care of. Screw Dumbledore! And Remus...he liked Remus, but the person he'd talked to a few times and had TRIED TO EAT HIM, even if it wasn't his fault, didn't measure up against the man who had, half-mad, tracked him down to Privet Drive before he'd gone to Hogwarts, even though he'd broken out specifically for revenge, who had ran into his room every morning that summer, so happy to see him. Even if Sirius hadn't known what was going on when he wasn't on his potion, he still loved Harry.
And that was why Harry was barricaded in the Room of requirements, ignoring the banging on the door and preparing to do a dangerous, and illegal, ritual. Who cared about some sliver of his soul/ He wanted his family back.