What did Snape know, and When did he know it?

[houyhnhnm102]

No: HPFGUIDX 169372

Mike:

> As I said above, it doesn't sound like Snape needed 
> to be anywhere near 100% in the loop, the knowledge 
> that Peter led LV to the Potters seems to have become 
> common knowledge amongst the DEs *after* the LV's 
> downfall. Likewise, the knowledge that Sirius Black 
> was *not* a fellow DE also seems to be common knowledge 
> among the DEs. At least Sirius Black's cousins Bella 
> and Narcissa would know this and, given that Snape and 
> Lucius were thick as thieves, I would expect that Snape 
> also knew this. 

houyhnhnm:
  
I don't see any evidence for this view of the openess of 
the DE sub-culture even pre GH.  Post GH, an awful lot of 
them seem to have been claiming to know nothing.  

Mike:

> So, why didn't Snape tell this to Dumbledore? 

houyhnhnm:

I snipped the rest, because *if* Snape knew that 
Peter Pettigrew was betraying the Order for a year 
and he withheld that information from Dumbledore, 
then there is no need for a convoluted theory to explain why.  

If Snape knew that Pettigrew was a Voldemort spy and 
failed to tell Dumbledore, then he is not only guilty 
of the Potters' deaths, but also those of Marlene 
MacKinnon, Benjy Fenwick, Edgar Bones, Caradoc Dearborn, 
Gideon and Fabian Prewett, Dorcas Meadows, and who 
knows how many nameless ordinary witches and wizards.  
Voldemort was winning at the time he went to Godric's 
Hollow, and who knows how much of that was due to the 
fact that the WW's anti-Voldemort guerillas were being 
decimated from within by a traitor. 

Why did Snape keep that knowledge to himself? Because 
he's evil, evil to the bone, Voldemort's man through and through.

And Dumbledore is an old fool.