I'm inclined to agree with you Unraveller. While I'm not saying it was right, what would have happened if Snape had not killed Dumbledore? If he had joined Dumby then, they both might have died. But now, he is in a perfect position to cause havoc in Voldy's ranks, and is also in a great position to leak info to the Order. And remember how Snape mentions how it would be hard to fool the dark lord, with him being a superb occlumence? Well, Snape is also a superb occlumence.