Lama, Snape swore an "Unbreakable Vow" to Mrs. Malfoy that if Draco fail in his task from Voldemort then Snape would finish it. If Snape didn't kill Dumbledore he would of broke this vow,(Don't know what would happen if you broke one but it probably can't be good). I truely believe Dumbledore is dead and I believe Snape is still on the good side even though Harry may not believe it. Snape has a role to play in defeating Voldemort and Dumbledore knew this. I think he froze Harry to prevent him from stoping Snape which would of in turn stoped Snape from completing his "Unbreakable Vow".
iirc draco's task was to get that portal working...(i'm going off reading this the day it came out, and i read the book in one day, so i might have it wrong <img src="/images/graemlins/smile.gif" alt="" /> )
but even if it was to kill dumbledore, snape mearly stepped in...so draco techniqely didn't fail...he never tried...snape "did" the deed (or so we are lead to believe) so as far as the vow goes draco and snape are in the clear...i.e draco hasen't attempted yet...so snape doesn't have too...but in the end dumbledore "appears" dead so voldemort is happy anyway....
all i know is there is no way that snape is bad....because after dumbledore's supposed death harry is suddenly free....whats he do....he goes right after snape....snape easily blocks anything harry can do (and we get another reminder of the silent spells)....
if snape was a bad guy then why not just freeze harry (like dumbledore has done iirc twice now to harry) and take him right to voldemort?
there is no way snape wouldn't have done that...if that was what he wanted too...dumbledore dead, and potter at voldemorts feet...victory..so why not?
imo dumbledore had to be killed by snape so voldemort would trust him....is snape the spy or double-agent?
someone has to destroy all the horcruxes so voldemort can be killed....
another thing i wonder about (and i have to go and re-read all the books) is in the Order of the Phoenix when sirius is killed he is kit by the fireball, and blown thru that doorway.....
earlier when harry was stumbling around he came across that and it gave me the impression of "the afterlife" so why a doorway...if the dead are on the other side, then can the living go get them? or can the dead leave...imo something will happen with that...