That's all part of it, though. The only people who knew that Draco had disarmed Dumbledore on top of the tower was Harry, Dumbledore, and Draco. Of course, Harry did not tell Snape anything after he killed Dumbledore, Draco obviously did not say anything, and I'm positive that Snape did not know that Dumbledore possessed the Elder Wand. Dumbledore shared the fact he possessed the Elder Wand with no one. The only person who knew he had won the wand from Grindlewald, was Grindlewald and Voldemort.
I know there are very big reasons why Dumbledore did not share that information with anyone. He wanted to make sure that the wand disappeared. I'm surprised he did not put it away and hide it or destroy it. Even if it were more powerful than any other wand, he did not intend to use it as such. Sure, he fought Voldemort with it in the DoM, but as we found out, he had never killed anyone with it and he obviously did not intend to kill Voldemort, only defend the situation.
Maybe if Snape had any of this knowledge he might have survived. He obviously knew that Voldemort took the wand from Dumbledore's tomb. Snape might have been ignorant of the importance of the wand and why it was that Voldemort wanted it badly enough to break open Dumbledore's tomb. But had he any of the knowledge that Dumbledore kept secret, might he have survived?
Dumbledore had his reasons for all the secrets and I hate to say that Harry and Snape were both pawns in his plans, but the outcome was for the greater good, I suppose.
I had said a month before the book came out after I had read the whole series twice through. I felt the main message was love. Then I thought about what DD said to Harry about telling VM about the prophecy was the greatest regret of Snape's life. I didn't think he knew her though. That suprised me. Anyway the locket was also a thought of mine when I read R.A.B. She kept making slights about Regulus in either four (G.O.F.) or five (O.O.T.P.)