What turned Snape (Was: JKR site update SPOILERS)

[Eddie]

No: HPFGUIDX 158903

> dumbledore11214:
>
> Interesting, very interesting. I found two things in the rumours site 
> to be especially curious.
> 
> " Snape was hiding under the Invisibility Cloak on the night the 
> Potters died.
> No, he wasn't."
>
> Alla:
> What I find curious is that in answering first rumour she does not 
> say that Snape was not there, only that he was not there under 
> Invisibility cloak

Eddie:
I think this touches on a question I've been having: What turned Snape
against Voldemort?  (If you think Snape was _NOT_ turned, then
rephrase the question as "What excuse did Snape give Dumbledore as to
why he turned against Voldemort?")

My literary sense is that the dramatic tension would be hightened by
Snape being turned by some event(s) he witnessed at Godrick's Hollow
on the night of Harry's parents' deaths.  Some have suggested that
Snape was in love with Lily, and was horrified that Voldy killed her.
 Yet another reason to have lots of anxiety about Harry.

But if my memory serves me right, Snape returned to Hogwarts BEFORE
the Godrick's Hollow incident.  What excuse would Snape have given
then?  (I think Snape tells Narcissa and Bellatrix that he was already
spying on Dumbledore by that time.)

Sorry if this has been answered in other threads, but my search didn't
find it.  At least not this exactly.

TIA,
Eddie