He could have told the things Dumbledore was up to to Bella and Narcissa or any of the other Death Eaters for that matter. Have you ever thought of this at all? What if the Dark Lord was getting suspicious of Snape's activity at Hogwarts? Dumbledore wouldn't want that to happen, so he had Snape kill him, to make it look like he was plotting against Dumbledore the whole time.