PHP: Snape learns that Malfoy is going to have to kill Dumbledore, Snape travels back to Dumbledore and tells him about Voldemort and his plan. Dumbledore tells Snape to lie low at his house for a bit, when Narcissa and Bella come and visits him. Narcissa tells him about her son Draco, and Snape takes the Unbreakable Vow, so it wouldn't be too suspicous, so then he goes back to Dumbledore and tells him about the vow. He makes him promise that he has to do whatever he says, which was killing him. Snape tried to back out, but Dumbledore said that he agreed, and that he would have to see it through. Later when Harry and Dumbledore went to the cave, Dumbledore knew what the potion would do, so he drinks it on purpose, so he would be weak, and so he couldn't defend himself against the curse. Back at the castle he confronts Malfoy, and stalls so Snape would come down, and kill him himself. Which concludes that Dumbledore valued other peoples lives more than his own. He had saved two lives for the price of one.