This file is just like so many others out there: A compilation of bits of pieces that don't have a full story behind, free to a good home.
Standard disclaimers apply: I don't own anything you recognise, any OCs are mine.
Idea #1: If Harry had asked exactly why he had to go back to the Dursleys. Also; the old Fanon cliche.
"Blood wards? So you mean to tell me that there's nothing at all stopping Riddle himself strolling up to the Dursley's front door?"
"Whatever do you mean, Harry?" Dumbledore's ever-twinkling eyes lost some of their luster.
"March '95. Bone of the father, flesh of the servant, and, listen closely, blood of the enemy."\
Idea #2, much like the first; Why Petunia can be a giraffe-necked, whey-faced, dark haired woman, while Lily is a fiery redhead.
"Blood wards? What blood?"
"Whatever do you mean, Harry?" Dumbledore's ever-twinkling eyes lost some of their luster.
"Have you even seen a picture of my mother next to Petunia? Or a family portrait? They look nothing alike. Mum was adopted."
Idea #3: Well, he was the Dark Lord's equal.
What most people seem to forget is that you need a Reason to go dark. What motivates someone to jump from being just another wizard with a dark magic addiction, to a terrifying monster willing to turn the world upside down and leave it a gibbering wreck in their wake.
And quite frankly, this was the last straw.
He would not let this stand. He would not allow this. Not now. Not ever.
If it meant walking over the ashes of half of Britain, then he'd say let the world burn. If it meant killing the minister in cold blood, he'd be the first one to conjure a knife. If it meant becoming a worse monster than Voldemort could ever dream of being... Well, so be it.
If Voldemort was a little smarter:
"You thought I'd seriously try a killing curse? Again? I'm a Dark Lord, not stupid, and it worked out for me so well last time." He said sarcastically, before turning to his pet snake. "§Kill§"
If Voldemort is a person preaching hatred and pureblood supremacy, what's his equal and opposite?
I do not discriminate by race, parentage, skin colour, or house affiliation, whether a person's parents had magic, or whether they're even human at all. If that's not neutral, I don't know what is. That being said, I don't preach forgiveness of the guilty at the expense of the innocent either.
You, Dumbledore, claim to be of the 'light', when your policies and practises let the guilty, the dark, the rapists and murderers go free, while the innocent suffer at their hands time and again. Your professed aims are ineffective, because you serve to demonstrate that the worst of crimes justify nothing more than a slap on the wrist. I say let the punishment fit the crime, and suffer not the unrepentant their freedom to re-offend.
The Real Reason Harry's not allowed to stay with Sirius:
Sirius is an immature man-child, whom sees nothing wrong with trying to feed a fellow child to a werewolf - his friend - which would've resulted in his friend's execution. It could be argued, though, that his incarceration and prolonged exposure to Dementors has left him slightly unhinged in the aftermath, so any later actions may have mitigating circumstances. That's still no excuse for trying to feed Snape to Lupin.
Another Sirius Idea
No one would accuse Sirius of being overly religious. Oh, he went to church occasionally, but on the other hand, he had stolen a couple of times from the collection plate for beer money. Then there were those nuns he slept with, and that priest he set on fire for calling him a sinner, and that time he took a piss in the baptismal pool... And his deflowering that set of twins the night before they were to be married...
And a point against all those Serevitus fics:
Severus, on the other hand, is a petty, vindictive man that bullies and belittles children, abusing a position of authority, showing little or no empathy, employs poor teaching methods, and - most tellingly - would have cheerfully fed an innocent man to dementors. (By which I'm talking about Remus, more so than Sirius.) He was, after all, in the Shrieking Shack - under the invisibility cloak - while the background of the Marauders was being explained.
All else known, Severus was a Death Eater, unrepentant and unremorseful - until Lily Potter was a target - therefore it is highly likely that he was already aware that Peter Pettigrew was a fellow marked Death Eater.
If Harry was a little less forgiving (TWT, second task):
"Why do you have Mr. Krum's hostage, Mr Potter?"
"Wait, Krum's Hostage? Who was mine then? Don't get me wrong, Cho's attractive and all, but she's Cedric's girlfriend, and I'm afraid I've never met the blond girl."
Dumbledore frowned. "Your best friend, of course..." Harry simply looked puzzled. "Mr. Weasley."
Harry couldn't contain his mirth. After he finally stopped laughing, and gasping for breath, he exclaimed: "What?! I did rescue my best friend, the only one that stood by me. Ron's a lazy, jealous git who refused to believe me when I told him I didn't enter. You need to get a mind healer to check you out if you thought that Ron would be 'what you'll sorely miss,'" Harry shook his head. "Fancy that, my Gillyweed's run out. And it was an hour's worth too. 'But past an hour – the prospect's black, Too late, it's gone, it won't come back.' and good riddance."