i dont think (i'll call him Mr X) is dead.they only saw him lying there, but they never saw any part of Mr X's body being buried.it was covered all the way.and who has heard of an avada kedavra spell causing someone to be flung away?normally its a green flash and the victim just dies on the spot.
what i believe is snape shouted Avada Kedavra but instead used a non-verbal spell to (maybe) stun Mr.X. Mr X flies, hits the ground and sustains injuries, and thats why we hear Fawkes singing- healing powers remember?
but its all just theories..
meh. no time for new sigs atm