Shouldn't it have been Harry Potter and the Seven (Eight?) Horcruxes? Why Deathly Hallows? Maybe this is Rowling's way of highlighting something that is not so obvious.
In the chapter King's Cross, there is a significant part of the conversation, where Dumbledore tells Harry about his trying to unite the Hallows to conquer death.
Dumbledore turned his whole body to face Harry, and tears still sparkled in the brilliantly blue eyes.
'Master of death, Harry, master of Death! Was I better, ultimately, than Voldemort?'
'Of course you were,' said Harry. 'Of course - how can you ask that? You never killed if you could avoid it!'
'True, true,' said Dumbledore, and he was like a child seeking reassurance. 'Yet, I, too, sought a way to conquer death, Harry.'
'Not the way he did,' said Harry. 'Hallows, not Horcruxes.'
'Hallows,' murmured Dumbledore, 'not Horcruxes. Precisely.'
This fits in well with their conversation about choices at the end of HP 2.
So we have here a triangle with Harry, Dumbledore and Voldemort at the three vertices. Both Dumbledore and Voldemort sought to conquer death. The paths they took, were, however, significantly different.
Voldemort chose to immortalize himself by making Horcruxes, living or non-living beings into which was incorporated a piece of his soul. He had to kill to be able to split his soul every time. Reminds me of an old fairy tale where a rakshasa (demon) could not be killed because he had lodged his soul in a bird, which was then kept under layers of security, and the protagonist had to breach each level of the protection to get to the bird, kill it, and so kill the rakshasa. The megalomania of the demon comes across in such a story, where there is complete disregard for the safety of or any pain suffered by anyone else. The deaths are deliberate.
Dumbledore, too, dreamt of conquering death. It was initially a nebulous wish, together with his teen-hood friend, Gellert Grindelwald. It begins with a common dream of conquering the world, of being the top-dog, so to say. Events that follow lead to a separating of ways for the two friends.
When Dumbledore defeats Grindelwald (now a feared dark wizard) much later, he wins the Elder wand, the first Hallow, from him. When he finds that James had the Invisibility Cloak, the third Hallow, he realizes that the legend of the Hallows is true. Consequently, he is tempted by the ring containing the second Hallow, the Resurrection Stone, and attempts to put it on. The ring is cursed by Voldemort, and it almost kills Dumbledore.
Note, here, that Voldemort had held in his hand a Hallow, and had turned it into a Horcrux. We realize that he had no knowledge of the Hallows, as he had been raised by muggles, and was later likely to be disdainful of 'mere children's stories' and anything to do with emotion.
Dumbledore, though, had never united the Hallows, though he possessed one or two of them at all times since obtaining the Elder wand. He had given the Invisibility Cloak away to Harry before he got the Resurrection Stone.
Harry, the third corner of the triangle, had never considered the possibility of doing either and conquering death. He, however, did possess all three Hallows after defeating Draco while escaping from Malfoy Manor. As Draco had overpowered Dumbledore on top of the Astronomy tower, he had won the allegiance of the Elder wand, and Harry won it on overpowering Draco.
So although Voldemort tried to kill him using the wand that he had forcibly removed from Dumbledore's grave, the wand gets snatched from his hand by Harry's signature Expelliarmus, and flies to its true owner, turning in an arc to hit Voldemort instead with his own killing curse.
The Invisibility Cloak belonged to him by rightful inheritance, and the Resurrection Stone had been bequeathed to him by Dumbledore. So, at the point where he walks into the Forbidden Forest, he has united all three, making him Master of Death.
The most important point to note here is that, he has not got any of these by killing or destruction. Even his overpowering of Draco had been an Expelliarmus in defence. He did not aspire to conquer death, and so he did, when he had all three Hallows.
Conquering death here could be looked at in two ways:
1) He had been frightened for his life until then, even until he finally walked into the forest.
"He felt his heart pounding fiercely in his chest...Terror washed over him as he lay on the floor, with that funeral drum pounding inside him. Would it hurt to die?"
When he opened the Snitch and turned the Stone over, his dead loved ones appear and reassure him. This part is reminiscent of the end of Dickens' A Tale Of Two Cities where the poor seamstress requests Sydney Carton to hold her hand till the end.
"You'll stay with me?"
"Until the very end," said James.
Harry was no longer frightened of death and walked to face his death with a sense of detachment.
2) Now that he had the three Hallows together, he no longer could be killed. Voldemort's Avada Kedavra in the forest selectively destroys the Horcrux bit in Harry. That was the fussing mass of humanity in the bundle behind the bench at King's Cross. The killing curse sends Harry, too, to the place in transit, but he has the option of coming back, and lives.
Dumbledore had the right idea in HP 1 when he hid the Philosopher's Stone in the Mirror of Erised.
"Only a person who wanted to find the Stone -- find it, but not use it -- would be able to get it. That is one of my more brilliant ideas. And between you and me, that is saying something."
Choosing the Hallows over the Horcruxes is the fork in the road taken. And that has made all the difference, to quote Robert Frost.
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