I am starting to wonder about the potion (poison) DD drank in the cave. He said "it wasn't a health drink" and it obviously weakened him badly.
What if DD realized that there was no antidote for the potion and, realizing he was going to die anyhow, mangaged to arrange things so that his death would have the greatest benefit for the good side?
Frankly, I wondered if it somehow gave the appearance of his being dead, when in truth he wasn't. I'm still wondering about the strength of Snape's curse, and if he just wanted to make it look to Malfoy like he truly killed D so that Malfoy could go back to his family, and the Dark Lord, and say he witnessed it, thus cementing in Voldemort's mind that Snape was still 'his man', even though he wasn't.