First, I present facts, and first-order analyses. Rereading appropriate bits of the text will corroborate these things. If anyone can find anything that contradicts any of this, I would be interested to hear it. Tenses may wobble a bit. For this I apologize, but only a little.
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- Voldemort assigned Draco to kill Dumbledore. As of the summer before, Draco was having nough trouble with it that Narcissa pled with Snape for help. Snape agreed, and swore an oath on his life. After he had agreed, Narcissa specified the terms to include that he would finish the job if Draco could not. As of that summer, Snape knew that the year would end with either himself or Dumbledore dead.
- Dumbledore knew that the position of DADA was cursed, such that any who held it would not be with the school past the year they held it. He permitted Snape to hold the position anyway (something Snape had craved for many years, but never gotten, for just that reason.)
-- Dumbledore and Snape both knew at the end of the year, one way or the other, Snape would not have the protection of Dumbledore or Hogwarts. The both also knew that this meant that Snape would have to fall back on Voldemort both to preserve his cover and to protect him from his many and varied enemies.
- Throughout the year, Our heroes kept coming across evidence that Snape and/or Draco were plotting *something* and kept bringing it to Dumbledore, and Dumbledore kept saying, in essence, "Yes, yes, I know all about this. It's nothing to worry about."
- Dumbledore acknowledged to Harry that he was no longer the wizard he once had been. He wilingly accepted permanent and severe damage to one hand as the price of destroying one of the horcruxes, and counted it well spent. He willingly accepted further damage in drinking from the goblet - damage that he knew would be severe and likely permanent as well - as a price of acquiring another horcrux.
-- from this I can only conclude that Dumbledore had ceased to consider himself as a long-term resource, and was instead trying to spend himself as effectively as possible in the short term.
- Dumbledore knows that Voldemort has a very serious thing in for him personally. He knows that one of the big things that was keeping him alive was Voldemort's fear of his personal power. He has to know that once he burns too much of that personal power, Voldemort *will* kill him. In sacrificing his hand to destroy the Horcux he knows that he has signed his own death warrant.
- Draco was having serious moral and emotional issues with the idea of killing Dumbledore. Snape knew this.
- Dumbledore has always had a thing about believing that people can be redeemed.
- Snape spent his time as DADA prof *actually* *teaching*. To the absolute best of his ability he taught them how to protect themselves against the coming storm. In his final confrontation with Harry (during which he had the upper hand the *entire* *time* and probably could have killed Harry or at least caused him grevious harm had he wished to) he instead spent precious moments *educating* the boy, emphasizing the tactical importance of learning how to cast wordlessly, spending his few remaining instants imparting as brutally effective a lesson as he could manage before completing his escape.
- In the rooftop confrontation, Dumbledore was toast even before Snape got there. He was effectively incapacitated, and surrounded by sworn enemies and Draco. There were more than enough death eaters to take him down at any moment - the only reason he hadn't died yet was that they were waiting for Draco to administer the killing blow. As soon as Snae sees the situation, he knows it's going to end one of three ways. Draco might finally drag together enough of whatever he needs to fire off an Avada at a man he's known and respected (if not necessarily liked) for years, largely demolishing his chances at redemption. The Death Eathers might grow tired of waiting, and finish the jobs themselves, thus doing serious harm to Draco's ability to survive in Voldemort's court, and quite possibly killing Snape through the vow. Finally, Snape can storm in there, pretending impatience, do the old man in himself (thus, incidentlly, ending his pain) and in so doing get a great deal *more* security for himself in Voldemort's court while remaining able to minimize the damage to Draco, and picking up some quite large and potentially very useful favors from Narcissa. Guys, this one isn't a betrayal. It's a no-brainer.
The only conclusion I can come to, based on the evidence, is that Snape told Dumbledore what was going on with Draco, Narcissa, and the vow relatively early on. Dumbledore decided that it was his time to die, and started to sacrifice himself on the installment plan. They both knew that Snape would not be able to stay in Hogwarts without Dumbledore to protect him, so Snape got to enjoy his last year as DADA (and was, arguably, the best DADA teacher they've ever had.) Snape continued to work on Draco, trying to keep him redemption fodder while looking like he was helping him, and giving Dumbledore regular updates. I suspect that Dumbledore had at least half made the decision to die *before* the vow, and Snape knew it, but I don't have strong enough evidence to state it outright.
It's true, Dumbledore and Snape made some harsh choices here, but they'd done things like that before - Snape especially. To my eyes, they were both serving the light as best as they were able.
[spoiler space (hi, blackaeronaut!)]
[/spoiler space]
- Voldemort assigned Draco to kill Dumbledore. As of the summer before, Draco was having nough trouble with it that Narcissa pled with Snape for help. Snape agreed, and swore an oath on his life. After he had agreed, Narcissa specified the terms to include that he would finish the job if Draco could not. As of that summer, Snape knew that the year would end with either himself or Dumbledore dead.
- Dumbledore knew that the position of DADA was cursed, such that any who held it would not be with the school past the year they held it. He permitted Snape to hold the position anyway (something Snape had craved for many years, but never gotten, for just that reason.)
-- Dumbledore and Snape both knew at the end of the year, one way or the other, Snape would not have the protection of Dumbledore or Hogwarts. The both also knew that this meant that Snape would have to fall back on Voldemort both to preserve his cover and to protect him from his many and varied enemies.
- Throughout the year, Our heroes kept coming across evidence that Snape and/or Draco were plotting *something* and kept bringing it to Dumbledore, and Dumbledore kept saying, in essence, "Yes, yes, I know all about this. It's nothing to worry about."
- Dumbledore acknowledged to Harry that he was no longer the wizard he once had been. He wilingly accepted permanent and severe damage to one hand as the price of destroying one of the horcruxes, and counted it well spent. He willingly accepted further damage in drinking from the goblet - damage that he knew would be severe and likely permanent as well - as a price of acquiring another horcrux.
-- from this I can only conclude that Dumbledore had ceased to consider himself as a long-term resource, and was instead trying to spend himself as effectively as possible in the short term.
- Dumbledore knows that Voldemort has a very serious thing in for him personally. He knows that one of the big things that was keeping him alive was Voldemort's fear of his personal power. He has to know that once he burns too much of that personal power, Voldemort *will* kill him. In sacrificing his hand to destroy the Horcux he knows that he has signed his own death warrant.
- Draco was having serious moral and emotional issues with the idea of killing Dumbledore. Snape knew this.
- Dumbledore has always had a thing about believing that people can be redeemed.
- Snape spent his time as DADA prof *actually* *teaching*. To the absolute best of his ability he taught them how to protect themselves against the coming storm. In his final confrontation with Harry (during which he had the upper hand the *entire* *time* and probably could have killed Harry or at least caused him grevious harm had he wished to) he instead spent precious moments *educating* the boy, emphasizing the tactical importance of learning how to cast wordlessly, spending his few remaining instants imparting as brutally effective a lesson as he could manage before completing his escape.
- In the rooftop confrontation, Dumbledore was toast even before Snape got there. He was effectively incapacitated, and surrounded by sworn enemies and Draco. There were more than enough death eaters to take him down at any moment - the only reason he hadn't died yet was that they were waiting for Draco to administer the killing blow. As soon as Snae sees the situation, he knows it's going to end one of three ways. Draco might finally drag together enough of whatever he needs to fire off an Avada at a man he's known and respected (if not necessarily liked) for years, largely demolishing his chances at redemption. The Death Eathers might grow tired of waiting, and finish the jobs themselves, thus doing serious harm to Draco's ability to survive in Voldemort's court, and quite possibly killing Snape through the vow. Finally, Snape can storm in there, pretending impatience, do the old man in himself (thus, incidentlly, ending his pain) and in so doing get a great deal *more* security for himself in Voldemort's court while remaining able to minimize the damage to Draco, and picking up some quite large and potentially very useful favors from Narcissa. Guys, this one isn't a betrayal. It's a no-brainer.
The only conclusion I can come to, based on the evidence, is that Snape told Dumbledore what was going on with Draco, Narcissa, and the vow relatively early on. Dumbledore decided that it was his time to die, and started to sacrifice himself on the installment plan. They both knew that Snape would not be able to stay in Hogwarts without Dumbledore to protect him, so Snape got to enjoy his last year as DADA (and was, arguably, the best DADA teacher they've ever had.) Snape continued to work on Draco, trying to keep him redemption fodder while looking like he was helping him, and giving Dumbledore regular updates. I suspect that Dumbledore had at least half made the decision to die *before* the vow, and Snape knew it, but I don't have strong enough evidence to state it outright.
It's true, Dumbledore and Snape made some harsh choices here, but they'd done things like that before - Snape especially. To my eyes, they were both serving the light as best as they were able.