My impression on finishing it, on a gut feeling level, was that it didn't have that much substance, seemed shallow somehow. Not that it wasn't fun, but ... well, as I said, before judging I'll have to read it again

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Mmmhh, Snape wanting to help Draco ... I hadn't thought about that angle much, but it could well be decisive.
My theory so far is pretty close to Barendd's - Snape works not for Dumbledore as such, but for the Order of the Phoenix and what it represents. In killing Dumbledore he sacrifices the White Queen, but he deems it
necessary for the chess game to be won. Maybe Dumbledore foresaw this, maybe not, either way he saw the necessity in the end.
I don't have the chess-player understanding of the situation at hand, but the end gambit planned might include:
Draco, the Pawn of Black, turning into a White Bishop through inside knowledge.
Harry, the Pawn of Black, reaching the enemy baseline through the Queen's sacrifice and becoming Queen of White himself, effecting the ultimate defeat of Black.