Brief explanation behind this question:
In Deathly Hallows, when Voldemort casts Avada Kedavra against Harry in the Forbidden Forest, the spell connects but (unknown to Voldemort at the time) does not kill Harry.
Two chapters later, Harry reveals himself and faces off against Voldemort in the Great Hall. He and Harry talk and Harry claims true ownership of the Elder Wand.
At this point, the best case scenario for Voldemort is that Harry is lying or mistaken, that he (Voldemort) is the true owner of the Elder Wand, and that Harry's attempted resistance will be of no consequence.
Yet Voldemort was using the same wand earlier and under the same conditions (having disposed of Snape, thinking to become the true master of the wand, before meeting Harry in the forest.) He should therefore expect the same result from the same spell, which clearly involved Harry not dying, even if Voldemort can't guess why. Why does he expect a second Avada Kedavra to work any better than the first?