“Oh, yes,” the boy said, “I could become a Death Eater and die as well; get killed by the aurors. That works, too.”.He was smiling as he said it, a twisted, bitter smile, but his eyes looked troubled.

First off, and I say this from experience, if you're going to archive somewhere, turn off the smart-quotes in Word. (”, versus "), because some of the archive sites won't recognize them.

Second, in this sentence, as in others, you want "That works, too." *SPACE* (delete the extra period) He was smiling.

has professor Flitwick taught you the patronus charm?”

I THINK canon is Professor Flitwick and Patronus...

I would fit, and everything would be wonderful. Ha!”His expression mocked the naive optimism of the small boy he had been.

Again, after the "Ha!" put a space.

You might want junediamanti to Britpick & Yorkshire check this...I'm CERTAINLY no expert on dialects of British English but she makes a fantabulous case for him being from Yorkshire, and it seems to me that you're trying to convey that's where he's from with some of his speech mannerisms, which I *also* love, but am absolutely no use to you whatsoever on verifying the accuracy of. :P

“Still, “he murmured,
“I feel surer of him than of Sirius Black.”

Your smartquotes are "off" here (I love them too but they're a bitch in archiving), and there's an unnecessary carriage return.

In fact, the headmaster suspected Sirius’s real loathing of Severus dated from the time Severus had defended Sirius’s small brother Regulus from Sirius and his gang.

I think in PoA, Sirius & Remus state that Snape & Sirius were at each other's throats from the word "go", and Regulus didn't come until at least a year later. Also, if my speculation is correct, someone had already hexed Snape's broom on what was probably close to his first flying lesson; assuming that the curriculum was the same, this should have been early first year.

All, even the insignificant - seeming Pettigrew,

I think you want to delete your spaces between the words and the hyphen, here.

I love how you're melding the spy/attempted murder/difficult childhood for Severus thing here. That's something I'm struggling with making realistic in my own fic so it's nice to see someone else trying to work it too.

As an aside, I have a rather firm opinion that Snape's Patronus was a King Cobra (accioslash rather quickly convinced me that it's NOW a Phoenix...!) *wink* Although I must say I like the Griffin better than the typical Raven, Spider or Bat. (*gag*)

Also, yeah. Sirius? WTF, O...even 15 years later, he says "Snape deserved it"...and happily would've used his *best friend* as a *murder weapon* against *another child*?!?! That is really just...not right.