(At the end of Harry Potter and the Order of the Phoenix Dumbledore repents of his habits of secrecy: many elements of Harry's own history Dumbledore has withheld from Harry, but he now sees that withholding as "an old man's mistake." He determines to be more open, and indeed in Half–Blood Prince reveals a great deal to Harry, and moreover encourages Harry to be open with his friends Ron and Hermione. But late in the book, when Harry demands to know why Dumbledore trusts Snape, the old man, after a moment of what appears to be internal debate, decides not to tell him. Given all the reasons he obviously has to be skeptical about Snape, whatever has so decisively earned his trust must be remarkable. We'll soon find out what it is.)
To which of his masters is Snape ultimately faithful? As is the case with all double agents, that will be determined by his final act. I believe that in the last book Snape will be confronted with a choice, a choice which will determine his loyalties once and for all. Until that moment even he will not know which side he is truly on. But at that moment, I believe, he will make his most important decision: he will sacrifice his own life to save Harry Potter's. And even this will not answer all the questions about him, for it will be impossible for us to know whether he offers himself because he hates Voledmort and believes in Dumbledore, or because it is the only way for him to even the score with his old much–hated rival James Potter, who once saved Snape's life. As Rebecca West wrote, there's no such thing as an unmixed motive.
Now, some people would say that Snape has already proved his perfidy by murdering Albus Dumbledore, atop a tower of Hogwarts Castle. But I am among those who believe—and I went on record with this view pretty early on—that Snape killed Dumbledore at Dumbledore's (silent) orders. The case, very briefly: (1) When Snape bursts out onto the battlements, he and Dumbledore just stare at each other for a moment. Which could mean nothing, but the chapter preceding that one and the chapter succeeding it emphasize Snape's extraordinary skill at Legilimency, that is, mind–reading. It would be surprising indeed if Dumbledore did not take advantage of that skill, which he himself also possesses. So I believe that when the two men seem to be just staring, they are in fact conversing. (2) When Dumbledore finally does speak, he says only, "Severus … please … "—at which point Snape raises his wand and pronounces the Killing Curse. Harry thinks that Dumbledore is pleading for his life, but if there is anything at all that we know about Dumbledore, it is that he has no fear of death (about which more later). And he does not say "please don't" but rather just "please." But why would he do this? Why would he order Snape to kill him? Well, that leads us to …
Draco Malfoy. At the beginning of Half–Blood Prince we learn that Lord Voldemort has given Draco a great and dangerous task, one which terrifies his mother Narcissa. Snape tells her that he knows what the Dark Lord has commanded Draco to do, though neither of them ever says what the task is, and I doubt that Snape does know—I think he's faking it. In fact, he's probably guessing that Draco has been asked to kill Harry Potter, which would explain his eagerness to take the Unbreakable Vow to finish the job if Draco fails. Not until we see Dumbledore and Draco on the battlement do we know what Draco has been ordered to do—kill Dumbledore—and I suspect that until that silent conversation with the Headmaster Snape remains in the dark as well.






