It didn't seem as though DD was too upset about being killed, especially since he petrified harry, and it was almost as if he had set it up that Snape was to kill him. (recall an argument between ol Snapey and DD somewhere in the middle of HBP, where Snape was complaining to DD that "this was going to far" or something like that. My guess is they were discussing when Snape was going to kill DD, fulfilling the unbreakable vow that DD knew Snape made for Malfoy.) If, in fact, DD is the last unknown horcrux, as unlikely as it seems, it would make his death, the fact that Snape killed him, DD's unyielding trust of the man, and DD's refusal to let Harry help him before his death, all come together. (Also, Voldy wanted something of Gryffindor's, and DD pretty much embodies what Godric Gryffindor stood for, and I still like the idea that he's a descendant.)
The only argument to this little theory of mine is that DD thinks Voldy used the snake as his last horcrux, and the snake is "unusually obedient" to Voldy, even for someone who is a parslemouth. That being said, DD is not a snake, he is, in fact, much wiser than a snake, and he has a whole soul.
Thoughts?