yarrow
What if killing Dumbledore was not only a Snape-Dumbledore plan but an Order of the Phoenix plan? Snape makes an unbreakable vow, knows Draco's weakness and that either one of them might die, tells the order.
I think Dumbledore would sacrafice his life for the safety of a student, even a Malfoy and potential Death Eater. Regardless, there's much more to his death than we know...
Note: I only read the 6th book once and a while ago, please feel free to correct any mistakes as I'm rereading it.
I don't believe this was an Order plan (not the full order at any rate) because some of the members' reactions were too real to be faked. Besides, it's far too risky to let every single member in the order know, there might be a spy, or some of them may not be clever/powerful enough to not let the information out if they are tortured or what not.
I do believe Dumbledore planned it though (whether he planned every detail right down to the exact minute or not I would know). Points pretty much same as everyone here who agrees.
How he kept on telling Harry about how he should be sacrificed if needed, since he isn't as useful or powerful.
How he didn't allow Snape the DADA position for such a long time, and then suddenly gives it to him, even when he knows of the one-year curse. (Did he decide on this after Snape figured out what he just Vowed to do?)
He trusts Snape. So Dumbledore does make mistakes, but I don't believe he would trust Snape without a very good reason.
And when you compare how he reacted when Draco was going to kill him and when Snape was going to kill him. For Draco, he was suffering (from that potion), but he didn't seem too worried or anything. He even kind of encouraged Draco. Whereas once Snape arrived, he actually pleaded with him, like there was something he desperately wanted Snape to go through with (kill him perhaps?)
Sorry for giving such unclear points, I haven't read the book in quite a while and don't have the book with me right at this moment.
sweatdrop Of course, I root for Snape, and without the book now, my brain probably conviniently forgot all evidence pointing towards the Snape-is-bad theory.