The reason DD couldn't kill Voldemort is in Harry. Harry has a piece of Voldermorts soul so if DD would have killed Voldemort then he could have just came back. So and Plus DD was ready to die before harry found out all this so Harry could not of gotten rid of the hocruex before DD died and nobody wanted to tell harry about the Hocruex in him until he understood everything about Voldmorts past. So that is why DD didn't kill or try to kill Voldemort