You heard it here first. Well, probably not, this was probably all over the Potter boards sometime Saturday morning.
Why Snape killed Dumbledore isn't important. What's important is that it sealed Harry's heart against him.
I expect that the outcome of the Harry Potter saga will hinge on Harry not killing Snape, or for that matter, maybe even Voldemort, because if he does in victory he will become worse than Voldemort.
"Come to the Dark side, young Potterwalker!"
It's all building up to the inevitable dilemma: he's going to have to decide between love and hate, between justice and revenge.