I'm unsure how we can argue that Snape is not "directly responsible" for Dumbledore's death, unless you (collective "you," here) are a member of the Dumbledore lives camp. Regardless of the circumstances, as Towerdweller points out, Snape does cast the AK. You may not think he is culpable for doing so, but he is responsible. However, that is a different question, really.

I find it interesting that JKR herself talks about Snape and his culpability. She says that because Snape has been loved, he is more culpable than Voldemort for his actions. When the author of the books says that, we have to listen. You may not agree with her, but it's her story, and what she says goes.

mlwl: Well, but likewise... aren't you attempting to say that he is innocent, regardless of any evidence presented to the contrary? This is why I love his character- I can re-read these books a hundred times, and the truth is that I don't think I'll ever know who Snape truly is until JKR gives us her final statement in book seven. And even then, I'm just really hoping that we get that final answer!

When I wrote “Half Blood, Half Prince,” my purpose was to lay out the facts from the books and evaluate the evidence afresh before drawing my own conclusions. I sincerely wanted to find out for myself if there was sufficient evidence to support a case that Snape is good. I did not write the essay to persuade anyone that Snape is evil, though that was my personal conclusion after reviewing the evidence.

We know that the argument was about some planned course of action that Snape had previously agreed to and was then having second thoughts about, we just don't know what that course of action was. The question I would have liked to have seen addressed is: why have the argument at all? If Snape was a loyal Death Eater, shouldn't he have continued with what his original plan would have been; pretend to agree with what Dumbledore wanted him to do and double cross him at the appropriate moment?

So Dumbledore had argued with Snape. In spite of all he had told Harry, in spite of his insistence that he trusted Snape completely, he had lost his temper with him . . . . He did not think that Snape had tried hard enough to investigate the Slytherines . . . or, perhaps, to investigate a single Slytherine: Malfoy? (HBP, pg. 407)

As Snape pointed out to Bellatrix, at Spinner's End, Voldemort had already ask all of her questions and if he had not been satisfied by the answers Snape would not have lived to attend that meeting. Therefore, that meeting did little more then to inform us of Voldemort's reasons for trusting Snape other than to inform us of the unbreakable vow. I would have liked to have seen a little less of a review of what those reasons were and a little more of a comparison with Dumbledore's reasons for trusting him.

Do we now think that we know why Dumbledore trusted Snape? There is a strong possibility that Dumbledore was merely reacting to Harry's discovery of who passed the information to Voldemort that made targets of himself and his parents; attempting to soften the fury that Harry felt as a result of that knowledge.

. . . how can you be sure that Snape’s on our side?” [asked Harry.]

Dumbledore did not speak for a moment; he looked as though he was trying to make up his mind about something. At last he said, “I am sure. I trust Severus Snape completely.” (HBP, pg. 549)

If that was the aggregate of Dumbledore's reason for his unwavering trust in Snape, then I would like to know why he kept is secret for so long. Was it because if others knew the reason for that trust, they wouldn't share it? I have a lot of respect for Dumbledore, for among other reasons, he can admit his mistakes and can warn people that his opinions can be a wrong as Humphery Belcher's who believed the time was ripe for a cheese cauldren; however, if it turns out that Dumbledore kept his reasons for trusting Snape secret in order to manipulate others into trusting him, I will lose all respect for him.