Answer:
He kills HE MUST NOT BE NAMED with the deathstick ( the most powerful wand)
Answer
SPOILERS perhaps.............................In the Goblet of Fire, Voldemort takes Harry's blood in order for himself to regain a body. Harry's blood contains Lily's protection and love for her son and now some of Harry's blood is in Voldemort. Thus if Voldemort tried to kill Harry, the spell would not kill the Harry part of Harry because the person holding the killing wand is partly Harry himself and Harry is protected. Thus the deathspell kills that part of Harry which is not Harry but a part of Voldemort. That is how I understand it. For more information refer to The Goblet of Fire, The Half Blood Prince and The Deathly Hallows.