Not saying I believe anything, but re-reading GoF, I noticed something.
First of all: Dumbledore never lies. But sometimes, he expresses things in a way that what he says is technically wrong but interpreted in a way that suits his intentions. Now, in Snape's trial, he says the following:
"Professor Snape joined our side and became a spy long before Voldemort's downfall. Today, he's no more a Death Eater than I am."
The obvious interpretation is that Snape became a spy for the order, but technically, it's not what he says. If you look at it from another perspective, namely that Dumbledore and Snape are both Death Eaters, it makes just as much sense. I really cannot picture him as evil, but why does he never say "Snape became a spy for the Order", only "Snape became a spy for our side"? Why not "he is not a Death Eater", but "he's no more a Death Eater than I am"? Strange things going on there..