There are Spoilers Here! If you clicked on the title for some unknown reason, turn back now!!!! This is a giant Spoiler-full post!
Alright. This is my opinion on the Severus kills Dumbledork situation. I'm going to go through the entire book, in chronological order... Prepare for a rather large post. I've stolen lots of it from other people, and have tweaked it quite a bit (I have an 11 page Word file of HBP arguements, so I don't know where I got each bit from), but yeah, I do agree entirely with the following opinion..
1/ First of all, we know Dumbledore was injured in the first two weeks of the holidays. He tells Harry that only Snape's great skill saved his life.
If Snape hates Dumbledore so much, why would he let him live? He had, to use only a little less skill, to not try quite so hard, and Dumbledore would have been lost, doubtless to Pomfrey's loud lamentations of how hard they all tried but the injury was too severe.
2/ Dumbledore's injury does not heal completely, throughout the entire book, Harry constantly notices it. I believe that Albus was slowly dying - that Severus could save his life at the time of the injury, but not heal him completely. And with
Dumbledore weak, possibley dying, he would want to maximise the leverage he could get off his potential death. Furthermore, he has always said that he does not fear death, that it's merely the next great adventure, etc.
3/ Dumbledore at last intervenes with the Dursleys. Why would he do it now? He's left Harry there alone and unloved for years. Why act now, when he could have waited until the end of Harry's schooling, or could have acted so many years earlier? I believe that this is evidence that Dumbledore knew he wasn't going to make it to the end of Harry's schooling. Dumbledore isn't a nice man. Oh, he has plenty of social graces, and people love him with his sweets and his eccentricities and his twinkling eyes. But a nice man would have been shocked and horrified and intervened immediately he knew what was happening to Harry at the Dursley's house. Dumbledore isn't a nice man - he is a great man, and there's a world of
difference. He has acted deliberately to allow Harry to be neglected
and abused - and there's a hint of physical violence as well as
emotional violence, when Harry is careful to stay out of Vernon's
reach after years of long practice - and he does not intervene. He
watches, until this year.
4/ Snape makes an Unbreakable Vow to Narcissa.
With Dumbledore expendable, here is the perfect opportunity to prove
beyond any doubt that Snape is Voldemort's man. Bellatrix was still
suspicious of Snape after his explanations, but the vow shook her
considerably. That Snape could make this vow and follow it through
puts him right behind the Dark Lord, and that's where to need to be to
wield a knife.
5/ Snape tells Bella he heard the plan that Draco would kill
Dumbledore from Voldemort.
This is not something he can lie about - Bellatrix is too close to
Voldemort not to be able to verify the information. I believe
absolutely that he went straight to Dumbledore with the information,
and they were able to use it, in the light of Dumbledore's health, to
make Snape's position unshakeable.
6/ Hagrid overhears Snape protesting to Dumbledore that 'he didn't
want to do it anymore'. Hagrid puts it down to Snape being worn-out
with his teaching duties, and Harry is far more suspicious.
Think how difficult this is for Albus and Snape. There would be many
moments in the saving of Draco, in the plan for Snape to go
deep-cover, when Snape doubted his own strength. When he didn't
believe that he'd be able to do this. When it hurt too damned much. I don't believe that Dumbledore displayed any behaviour at all that could indicate that he
doubted Severus.
7/ Dumbledore starts telling Harry everything about Voldemort, helping
him to understand Voldemort, but he tells him nothing about Snape.
He's discomfited when Harry demands answers, and he evades the
question with a piss-weak answer that Snape was sorry for his
involvement in the prophecy and the subsequent death of Harry's
parents, and then he tells him that he'll tell Harry more later.
Harry cannot know what is happening with Snape, and he cannot be
trusted. A Leglimancer like Voldemort will have no trouble stripping
any information or belief that Snape is a spy if Harry is caught. He's
not yet competent. And he cannot get over his hurdle of hating Snape.
And this is after Dumbledore has promised to tell Harry everything.
This is something he cannot tell, cannot risk, or it risks the
placement of Snape right where he needs to be.
8/ Dumbledore takes Harry with him to get the next Horcrux because
he's not sure he's going to be able to get back, and when they do make
it back, with Dumbledore grievously wounded, he's completely focussed
upon Harry getting Snape. When they are told of the Dark Mark above
Hogwarts, Dumbledore is anxious to get back.
Dumbledore begged Harry to kill him in the cave, desperate to die, but
Harry brought him through it. His obedience allows Dumbledore to do
what must be done, and he's trusting that Dumbledore is right. When
they return to Hogsmeade, Dumbledore is not looking for treatment,
he's knowing instead that his time is short and it's time has come for
them to play that hand. He is out of the game in one last, desperate
all-or-nothing gamble. And for that, he needs Snape. Snape is
essential to the game, and Harry's role in this play is over. Which
leads nicely to this following point...
9/ Dumbledore immobilises Harry
The desperation for Harry to get Snape becomes irrelevant when Draco
comes through the door. Here it is - the play to save Draco, and
failing that, to get the greatest leverage possible from Dumbledore's
death. Harry's active presence can be nothing but a hindrance, so he
is made an observer instead. And if Dumbledore was genuinely
interested in saving his own life, there is no possible way that he
would have wasted his time on immobilising Harry.
11/ Albus' death
Oh, the desperation of this moment. Four Death Eaters, Draco, Albus
and Snape. And this is it, the moment when Snape is in the crucible,
the last desperate play that will bring salvation through ruin, or
salvation and ruin.
"Severus..."
There is a moment when there eyes are locked - and Dumbledore and
Snape are both Legilimencers and Occlumencers. How is it not possible
that there is a passage of thoughts, of feelings, of memories between
them? There must be, and Severus' face is twisted with revulsion and
hatred. How can it not be? For himself, at his capacity to do this one
final thing. For Albus, for bringing them to this point and for
demanding this sacrifice of him. For everything about this terrible,
terrible moment of sacrifice and pain, desperation and determination.
And Severus strikes. Then runs, as he must.
It is widely believed that Dumbledore was pleading fo his life, but by now I should have proved that this does not make sense, that Dumbledore knew he would die, and was not afraid to, especially in that his death would save Snape, Draco, and the other Malfoys.
12/ When chasing Snape, Harry throws curse after curse at him. Snape
throws them all off, and taunts him.
Snape stops Harry from performing an Unforgivable. "No Unforgivable
Curses from you, Potter!" He blocks him, and he does not strike back.
"Blocked again, and again, and again until you learn to keep your
mouth shut and your mind closed, Potter!" and it's a sneer, and it's
total absolute truth. Someone strikes at Harry with what must be a
Crucius, and Snape stops it. Then Harry attempts to use Snape's own
curse against him, and Snape strikes, throwing Harry backwards, but
still, still not hurting him.
And then he calls Snape a coward, and Snape in this moment of
desperation breaks. Snape has just sacrificed everything in a moment
that surely must have been harder than dying - he has just killed the only one who ever trusted him, the only one who believed he could redeem himself.
He strikes once. Whether he would have struck again is a moot point,
because of the attack of Buckbeak.
I believe that this battle was an opportunity for Snape to take Harry
from the field and serve him up to Voldemort triumphant, as betrayer
and capturer for his Dark Lord. And I believe that because he didn't
take the opportunity, it is all false, and Snape is still saving
Harry, as he has done before.