Okay, first of all, as already mentioned Draco had no choice. And I think Dumbledore values his students lives more than his own. Which I came up with a theory. Dumbledore finds out that Malfoy was a Death Eater, so he stations Snape at his house, where Bella and Narcissa come. Since he already knew about Draco being a Death Eater, he was prepared to make the Unbreakable Vow. Since Dumbledore knew that Draco wouildn't be able to kill him, he had Snape, so both Malfoy and Snape would live. That would prove that Snape is good, and Dumbldore cares more about his students than himself.