blueshield

I believe that Dumbledore knew that this moment may very well arrive. He made it quit clear to draco that he knew what he had been upto. It was also made clear earlier on in the book that Dumbledore was well aware of the vow Snape had taken. Hadn´t hagrid overheard this conversation? Had Snape not said that Dumbledore was asking to much of him? I therefore believe that Dumbledoore had asked Snape to forfill his vow. That should the time come, Snape was to kill him, making sure Draco could not.
There are in my opinion two reasons. One being that Dumbledore did not want Draco to be harmed, neither did he want Draco´s family to be murdered, no matter how bad they are. Dumbledore was after all a loving man.
Secondly, I can only presume Dumbledore had a plan in case this scenario took place. His death was needed, but I don´t believe his story ends there. He would not have let himself be killed without taking measures and he knows all to well that the situation is so grave he can not just leave it as it is. His wisdom and helping hand are still too greatly needed.
I do not believe he has split his soul or anything like that, But somehow he has left a piece of himself, or at least a peace of his mind. I truely believe we have yet to say our true goodbyes to this wounderfull man.

I want to add one more thing. The moment when Harry was chasing Snape and called him a coward. It infuriated Snape. If he did indeed kill Dumbledore for the greater good, it would mean he would be one of the bravest of them all up untill now.
To be honest I don´t think I could cope with the idea that his death was pointless. I read somewhere else someone saying that he was the best charater in the book. I completely agree. Fictional or not, in our hearts, minds and souls, he has existed. He will be sorely missed.