I think that Snape is a jerk and a biased, obsessive bully, but he is definately not a coward(I still love him though

). I think that he will do whatever he has to do, especially if it is for his own interests, or to save his skin. Ultimately, I think that Dumbledore told Snape to kill him, as upon learning about the Unbreakable Vow, Dumbledore would have valued Snape and Draco's lives over his own.