my theory on the ending of HBP
Dumbledore wanted Snape to kill him. He realized that Harry would not be able to fufill his duty if he was still there as a safety net. He understood that as long as he were alive, Harry could lean on him of guidance. But only Harry can fufill the prophecy. He also knew not only that it was Draco's assignment to kill him but also that he could not. Snape and Dumbledore knew that Draco would not and only when the moment of truth approached did Draco realize it as well. Snape and Draco are both double agents. Draco lost his interest in becoming a Death Eater after he watched what had happened to his father.