VA/H=Mx13+RP? Snape's Culpability?

[pippin_999]

No: HPFGUIDX 147214

Pippin: 
> > According to Dumbledore, something changed
> > when Snape discovered that the enemies Voldemort was going to
> > attack as a result of his information were people that he knew.
> >
kchuplis: 
> Oo. Where is this? I just don't remember it. It always seems like 
> everything said in the books about Snape and that time is so vague. 
> Thanks!

Pippin:
HBP ch25 p549 US

"But he did not know--he had no possible way of knowing --which
boy Voldemort would hunt from then onward, or that the parents he
would destroy in his muderous quest were people that Professor
Snape knew, that they were your mother and father--"
...
"You have no idea of the remorse Professor Snape felt when he realized
how Lord Voldemort had interpreted the prophecy, Harry. I believe it
to be the greatest regret of his life and the reason that he returned--"
---

Harry interprets this as Snape returning after his parents were dead,
but we know from the Pensieve trial in GoF and Snape's conversation
with Bella that he was working with Dumbledore before Godric's Hollow.

Pippin