He knew Malfoy wasn't going to kill him, so I think he was playing for time in the hope that someone from the order would reach him.
Also, Harry is going to have to improve his duelling a lot if his is going to take on Snape and Voldemort. Snape deflected his spells easily.
But, on the point of Snape killing DD on his orders, there may be some evidence:
- when Harry is trying to hex him, although he could just be mocking him, he is still trying to get Harry to cast his spell silently. I think this is going to be very important as LV and DD never spoke an incantation during their duel in OotP.
Harry calls him a coward, which Snape doesn't like at all. It could be that if he is working on DDs orders, he is actually been incredibly brave
Why did DD stun Harry when he was underneath his invisibility cloak if he had not planned for this all to happen? There was no need to do it and Harry could certainly have helped.