"It is our choices Harry, that show what we truly are, far more than our abilities." (CS18)
"You fail to recognize that it matters not what someone is born, but what they grow up to be!" (GF36)
But we have a problem with Merope Gaunt, and therefore a problem with Voldemort by association. Read carefully this dialogue between Harry and Dumbledore:
"So we know that, near the end of her pregnancy, Merope was alone in London and in desperate need of gold, desperate enough to sell her one and only valuable possession, the locket that was one of Marvolo's treasured family heirlooms."
"But she could do magic!" said Harry impatiently. "She could have got food and everything for herself by magic, couldn't she?"
"Ah," said Dumbledore, "perhaps she could. But it is my belief—I am guessing again, but I am sure I am right — that when her husband abandoned her, Merope stopped using magic. I do not think that she wanted to be a witch any longer. Of course, it is also possible that her unrequited love and the attendant despair sapped her of her powers; that can happen. In any case, as you are about to see, Merope refused to raise her wand even to save her own life."
"She wouldn't even stay alive for her son?"
Dumbledore raised his eyebrows. "Could you possibly be feeling sorry for Lord Voldemort?"
"No," said Harry quickly, "but she had a choice, didn't she, not like my mother —"
"Your mother had a choice too," said Dumbledore gently. "Yes, Merope Riddle chose death in spite of a son who needed her, but do not judge her too harshly, Harry. She was greatly weakened by long suffering and she never had your mother's courage. (HBP-13)
A few things are noteworthy here. Dumbledore's question, "Could you possibly be feeling sorry for Lord Voldemort" poses a bit of a challenge to the "Voldemort as Symbol of Evil" theory that I have espoused. In short, I believe Voldemort is not redeemable, because, for literary reasons, he serves as a symbol of all evil in the Wizarding World, and therefore must be defeated, not redeemed. He is sub-human and has placed himself beyond redemption.
But it's almost as if Dumbledore is prodding Harry to find something in Voldemort to feel sorry for. I still stand by the "unredeemable Voldemort" theory, and I assume Rowling has to include some element of humanity into Voldemort's story in order to maintain the literary use of Voldemort as Harry's shadow. But perhaps the most important observation here is that this tension points to the great difficulty and paradox of the problems of evil and choice. Even though Rowling's lessons through Dumbledore are rather simple concerning choice, the way they play out is vastly complex, and it appears Dumbledore knows this well. (The quotes above do not negate or contradict the complexity of the problem of evil and choice; they make sense in their own contexts.)
The problem is here: How culpable is Voldemort? How culpable is Merope? Harry is outraged that Merope, given the choice, would not stay alive for her son, while his own mother willingly gave her life for him. But Dumbledore's response is instructive: "[D]o not judge her too harshly, Harry. She was greatly weakened by long suffering and she never had your mother's courage."
First, let's address the point that has been astutely made by Sword of Gryffindor commenter "Mrs. Weasley": Merope was indeed courageous in a lot of ways. Merope certainly endured a trying and difficult pregnancy in order to give birth and life to her son, which demonstrates at least some level of love and courage. That said, I don't think Dumbledore was being necessarily judgmental when he said that Merope's courage was less than Lily's. He's really trying to stop Harry from judging her harshly based on Merope's life circumstances. In short, he's saying, "Merope did what she could given what she became as a result of her abusive past."
And that leads to the key tension here: in essence, it is not as simple as making the right choices, regardless of what you were born; it is possible that someone else can make choices for you - choices that permanently damage your own ability to choose. Merope was unable to summon Lily's courage, not because she had two equally available choices to her and made the wrong choice, but because she had been severely abused, and for that reason did not have the courage to use magic to stay alive.
We can even back this up further in the Slytherin line: we at least know that Marvolo and Morphin were mentally ill (and we can probably be certain that others in the line were such as well) due to inbreeding amongst Slytherin descendants; in other words, their ability to choose between right and wrong was affected by the choices their ancestors made which resulted in genetic distortions. That raises the same question: If past Slytherins made choices that resulted in a mentally ill prodigy (for physiological reasons), to what extent were Marvolo and Morphin culpable?
We do have evidence that Rowling herself sees a difference in the extent of moral culpability from one person to the next, depending on his or her personal history. Here is a very telling answer to a question from the oft-quoted Anelli/Spartz interview in 2005:
MA: Oh, here’s one [from our forums] that I’ve really got to ask you. Has Snape ever been loved by anyone?
JKR: Yes, he has, which in some ways makes him more culpable even than Voldemort, who never has.
At the very least, we learn here that some choices are morally more consequential than others, not based on the options themselves (good vs. evil), but based on the person's own circumstances. The key to Rowling's answer is this: Because Snape has been loved and Voldemort has not, the option to choose good is more available to Snape than it is to Voldemort.
It's hard to know exactly where to go with this information, except to say this: Rowling is acutely aware that one person's evil choices do not affect only that person. This gets at some of the worst aspects of evil: that the choices of one person can effectively debilitate another from having the same resources of love, compassion, and goodness that others have. Some people have less capacity for choosing the good and not the evil, because people before them have made evil choices.
It is an extremely difficult tension: Voldemort needs simultaneously to be morally culpable for his choices and so excessively evil that he is beyond redemption. The fact that he was deprived of the proper resources of love and goodness by his Slytherin ancestry complicates the matter significantly. Given Rowling's views on the value and capability of children (she believes they're underestimated by adults), it's at least safe to say that Rowling would give more value to Tom's choices before he was 11 than most of us.
But it would be hard to say that Rowling has any other reason for giving us Voldemort's history pre-birth than to make us actually feel a little bit sorry for him - just a little! - at least his childhood circumstances. We surely feel sorry for Merope.
As far as Voldemort goes, horcrux-making is perhaps he best answer to the choice problem, how Voldemort can be simultaneously a morally culpable person and beyond redemption. His choice to make them - a choice that was not programmed in him by anything genetic - that sapped him of his humanity. In effect, he chose to become less than human, to give up his own humanity - and therefore hope of "redemption" - by creating horcruxes.
Rowling once again demonstrates that she is not dealing in over-simplified categories. The themes present in her magical world continue to be a parallel to our vasty complex Muggle world. There's no simple, "This is good, this is evil; this person is good because of this choice, and this person is evil because of that choice." Rather, evil, is all its absolute vileness, is presented as something that does not only destroy the person committing the evil, but the people around him or her as well.