Snape can't be Evil

[vlngrrl]

No: HPFGUIDX 134022

Forgive me if somebody has already suggested this, but I was just 
rereading chapter 2 of HBP, and I realized something.  When Cissy 
spoke to Snape about what Draco had been asked to do, Snape knew about 
this already.

Is it possible that Snape heard about Draco's task, and has already 
spoken to Dumbledore about it?  Dumbledore made Snape swear to use any 
means necessary to prevent Draco from killing him, much the same way 
as he made Harry swear to force-feed him that potion in the cave.  The 
reason why Snape could make the unbreakable vow was because he had 
already promised Dumbledore the same.  Killing DD would be essentially 
keeping his vow to DD AND looking good to Voldemort at the same time.  
Everybody wins.

Sarah (who doesn't really like Snape, but wants to believe he isn't as 
black and white as it may seem)