This is for me the biggest unresolved question of the Books. Dumbledore planned from the beginning that Harry should first destroy all Horcruxes and then should die by the hand of Voldemort (which leads to Piton telling to Harry that he must die) and that Harry should kill Voldemort as last thing. The point is: why? Harry since The Goblet of Fire was "immortal", namely since Voldemort took Harry's blood in his vein. Quoting the 7th book, chapter 35:
‘Precisely!’ said Dumbledore. ‘He took your blood and rebuilt his living body with it! Your blood in his veins, Harry, Lily’s protection inside both of you! He tethered you to life while he lives!’
‘I live ... while he lives? But I thought ... I thought it was the other way round! I thought we both had to die? Or is it the same thing?
‘He took your blood believing it would strenghten him. he took into his body a tiny part of the enchantment your mother laid upon ypu when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so do you’. And this all make sense with the prophecy.
So, it is clear that Harry would (or better COULD, if he chose to return back every time he died) live, even if he jumped from a cliff or someone killed him, till Voldemort was alive. Then, why not kill himself alone (or with the help of his friends) to destroy the part of Voldemort which is in him and then do the other things? Why is it so essential" that it is Voldemort to kill Harry?
The only reason I could find is the fact that Dumbledore wanted Harry to accept death so that he could fully handle all the Deathly Hallows in order to have the power to kill Voldemort (and yes, the Deathly Hallows in the books have only the function of making Harry strong enough to kill Voldemort, which in the end happened and was the reason Harry won, because Voldemort died by the Elder Wand as Harry was the master of it, since Harry took it from Draco's manor. Deathly Hallows have nothing to do with Harry returning to life or anything special, they are only power). To support this, in book 7, chapter 35 Dumbledore says:
‘If you laid hands on them, I wanted you to possess them safely. You are the true master of death, because the true master does not seek to run away from Death. He accepts that he must die’.
Still, Harry could have processed and accepted death even from other Death Eaters or other people, so I think that this is not the answer.