Snape, a Deatheater.

[pippin_999]

No: HPFGUIDX 163942


> Nikkalmati
> Yes, he does probably learn that Draco has backup and warns him off any  
> silly attempts in the future, but he doesn't accomplish all that much.  (It  is 
> listees who have figured out that Bella is involved, we are never told SS has  
> that idea, but I would not be surprised if it were true). 

Pippin:
Listees who read carefully know that Bella is involved because
Snape says so: "Ah....Aunt Bellatrix has been teaching you Occlumency,
I see." --HBP ch 15

Really, your theory has Snape figuring out what he agreed to so early
that it scarcely makes any difference, unless what you're saying is that
he wouldn't have agreed at all if he'd known. 

But then we should not only see something which we could later
recognize as the moment when he realizes what he's agreed to do,
we should see Snape trying desperately to undo his mistake. That
doesn't happen.


Nikklamati:
 We do  find  out  that Draco has refused to come see SS when told to 
and we see Draco walk out  on SS in a fit of anger.  Furthermore, SS 
and DD don't seem to learn  anything  more about Draco's task during 
the rest of the year than they knew at  Christmas!

Pippin:
But that's what the argument in the forest is about, seemingly. 
Dumbledore thinks Draco's plans can never come to fruition because
there's no way to get DE's into the castle. Snape thinks that Dumbledore
takes too much for granted. Dumbledore wins the argument, and 
Snape is told to continue his investigations in Slytherin -- not, one
assumes, continue them with Bella or in Hogsmeade or do anything
which would make Voldemort aware that Dumbledore knows
that Draco is plotting against him.

Pippin