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

[justcarol67]

No: HPFGUIDX 169369

Mike:
> And I take this one step further. The (possible) fact that Snape knew 
> that Sirius wasn't the traitor doesn't stop Snape from wanting Sirius 
> dead. And Snape doesn't have to agree with *everything* that 
> Dumbledore believes in, so it still doesn't affect his DDMness!!
>
Carol responds:

Thanks for that concession, Mike. If *you* don't think it affects his
DDMness, and you're not a DDMsnaper, then I have nothing to worry
about! :-)

Seriously, though, if Snape thought, as we know he did, that Sirius
Black tried to kill him when he was sixteen and that Black actually
did kill twelve Muggles and Peter Pettigrew, and if Snape *knew* (as
we know he did) that Black had gotten into Hogwarts, slashed up the
painting of the Fat Lady, gotten into Hogwarts a second time and
actually slashed Ron's bedcurtains with a twelve-inch knife, wouldn't
he have thought, with good reason, that Black was trying to kill
someone? And, given that he didn't know that Pettigrew was alive, much
less an Animagus posing as Ron's pet rat, wouldn't he have thought, as
everyone else did, that the person Black was trying to kill was Harry?

What else was he to think?

Carol, keeping her argument simple this time to make sure that this
one question is answered