Rowling knows what she's doing. She is an excellent writer and her plotlines are clearly worked out well in advance.
More importantly, she knows IMPACT. She knows how to hit the reader with a revelation, and hit them again while they're still reeling from the first one.
I knew Dumbledore had to die. We've all known it -- the refrain of "As long as Dumbledore's around, you're safe, Harry" has basically let us know that the old wizard had a bright red laser-targeting dot in the center of his forehead.
We all knew Snape was going to have to at least LOOK like he'd gone really bad, didn't we?
But the two events were still shocking in their extremity. Dumbledore, dying in a pathetic way, without even a dramatic finish, and Snape, doing the deed himself with a viciousness that left one breathless.
... And once more, I'm left wondering what tricks she's going to pull. That flare of Phoenix fire... the constant mentions and uses of Polyjuice potion... the fact that Snape is possibly the most accomplished Occulumens ever to live... the fact that Dumbledore clearly trusted Snape without reservation... the odd contradictions in the Potter background, the occasional strange hesitations by various characters discussing certain events...
... Is Dumbledore really dead? Yes, we know that after someone dies like Sirius there is no coming back... but if you are one of the most powerful wizards ever to live, and you have a Phoenix for your friend and familiar, and you KNOW that you're going to be killed under some specific circumstances, perhaps... perhaps there IS a way to live again.
... or, perhaps... it isn't Dumbledore who died at all, but Snape... in disguise... with Dumbledore, disguised as Snape, doing the (apparent) assassination.
Part of me even toys with the idea that Snape has been playing a role since the very first day we met him. What actual evidence do we have of Snape's version of the past? The strongest evidence is Snape's memories.
But SNAPE is a GREAT Occlumens. And it's stated clearly that one can make convincing false memories (from the example that Slughorn does it poorly). The other evidence is from other people IN THE ORDER OF THE PHOENIX -- all of whom could, and would, keep secrets or play roles in order to maintain the proper appearances.
What if Harry's father was influenced -- as we would expect -- by Lily? James and Lily Potter were apparently loved by everyone who knew them. Harry's behavior is what many people seem to expect of their son. Harry is the way he is DESPITE the terrible childhood he had. How likely is it, then, that James Potter was, or remained, the arrogant bastard we saw in those memories?
What if Snape hated/disliked James, not because James was actually a bastard, but because James got Lily? Snape is the sort of man who values people, oddly enough. His behavior towards Malfoy's mother shows this. Given this, he would be exactly the sort of man to risk everything to protect the child of the woman he loved. Even if it meant making that child hate his guts.
Dumbledore makes mistakes, yes. But his insistence on trusting Snape seems impossibly thin to justify, especially when the justification we hear is that Snape was sooooo sorry after James and Lily died. Thin... unless Dumbledore knew that Snape, rather than hating or just tolerating the Potters, felt very differently about them. That Snape cared very much for them, and didn't realize that he was about to get them killed.
Well, we shall see. But boy, what a tangled web she weaves.