If not DD then maybe the Sempra Sectum killed Malfoy ?
It said snape did the spell to heal Malfoy 3 times

1) to close the wounds .....not to save him.
2) to preform the spell to turn him int an Inferi.......would explain why malfoy is pail.....or maybe Malfoy is already dead?.......Goes back to what DD said," How can they kill you if you're already dead".
3) to make Harry's crux....if Malfoy wasn't dead before.

Just a thought outside the box.