"…It warns in this book how unstable you make the rest of your soul by ripping it, and that's just by making one Horcrux!"
Harry remembered what Dumbledore had said about Voldemort moving beyond "usual evil."
"Isn't there any way of putting yourself back together?" Ron asked.
"Yes," said Hermione with a hollow smile, "but it would be excruciatingly painful."
"Why? How do you do it?" asked Harry.
"Remorse," said Hermione. "You've got to really feel what you've done. There's a footnote. Apparently the pain of it can destroy you. I can't see Voldemort attempting it somehow, can you?" (6.71-73)
The matter of exactly how evil Voldemort is – more evil than anyone else has ever been, apparently – is one of the most frightening things about him. As Hermione notes, it's unlikely that he'll ever feel any remorse for any of the terrible things he's done.
"Your point about Wizard dominance being FOR THE MUGGLES' OWN GOOD – this, I think is the crucial point… We seize control FOR THE GREATER GOOD." (18.34)
Whoa, there – this quote from a teenage Dumbledore to teenage Grindelwald is shocking. However, it raises an intriguing question – what is "the greater good"? How, for that matter, do we determine what is good or evil? Why should the two former friends have split, but continued, each in his own way, to hold to similar ideals, but on different sides?
Three objects, or Hallows, which, if united, will make the possessor master of Death… Master… Conqueror… Vanquisher… the last enemy that shall be destroyed is death…
And he saw himself, possessor of the Hallows, facing Voldemort, whose Horcruxes were no match… Neither can live while the other survives… Was this the answer? Hallows versus Horcruxes? Was there a way, after all, to ensure that he was the one who triumphed? If he were the master of the Deathly Hallows, would he be safe? (22.19-21)
Here, Harry first figures out the real conflict here – Hallows vs. Horcruxes, one vision of mastering Death against the other. And, really, it's a matter of good vs. evil – the Hallows, as we learn later, require a true understanding of Death, while Horcruxes seek to artificially and unnaturally avoid it.
"I'd say that it's one short step from 'Wizards first' to 'Purebloods first,' then to 'Death Eaters,'" replied Kingsley. "We're all human, aren't we? Every human life is worth the same, and worth saving." (22.78)
Well said, Kingsley. He touches upon the essential difference between good and evil, or between Order of the Phoenix and Death Eater here – the Order stands for equality, while the Death Eaters represent prejudice and an inhuman and unjust elitism.
"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 to sought a way to conquer death, Harry."
"Not the way he did," said Harry. […] "Hallows, not Horcruxes." (29)
Again, the question of ends vs. means arises – if Dumbledore and Voldemort had the same goal in mind, does it even matter that they worked towards it in different ways? Harry reassures Dumbledore that it does… what do you think?
Professor Minerva McGonagall
"I shall expect you and the Slytherins in the Great Hall in twenty minutes, also," said Professor McGonagall. "If you wish to leave with your students, we shall not stop you. But if any of you attempt to sabotage our resistance or take up arms against us within the castle, then, Horace, we duel to kill."
"Minerva!" he said, aghast.
"The time has come for Slytherin House to decide upon its loyalties," interrupted Professor McGonagall. (30.63)
Slytherin House, led by Professor Slughorn, is forced to decide whose side it's on – the school's, or its most famous alumnus's. Professor McGonagall's ultimatum makes it pretty clear that the time for moral ambiguity is past.
"I regret it," said Voldemort coldly.
He turned away; there was no sadness in him, no remorse. It was time to leave this shack and take charge, with a wand that would now do his full bidding… Voldemort swept from the room without a backward glance… (32.121)
Voldemort's true depths of evil are really revealed by his callous murder of Snape, who he thought was his faithful servant all these years. He truly has progressed beyond "the usual evil" – he's past all human feeling.
"Hide them all, then," he croaked. "Keep her – them – safe. Please."
"And what will you give me in return, Severus?"
"In – in return?" Snape gaped at Dumbledore, and Harry expected him to protest, but after a long moment he said, "Anything." (33.129-130)
Here, we see the moment of Snape's shift from evil to good – his betrayal of Lord Voldemort because of Lily's endangerment demonstrates the power of his love over his desire to serve the Dark Lord.
"And his knowledge remained woefully incomplete, Harry! That which Voldemort does not value, he takes no trouble to comprehend. Of house-elves and children's tales, of love, loyalty, and innocence, Voldemort knows and understands nothing. Nothing." (35.26)
Again, the difference between Harry's understanding of the world and Voldemort's is made clear – Voldemort is completely consumed by his lust for power, and his ignorance of the things that actually make life good and worthwhile are what really demonstrate his true evil.
"You show spirit and bravery, and you come of noble stock. You will make a very valuable Death Eater. We need your kind, Neville Longbottom."
"I'll join you when hell freezes over," said Neville. "Dumbledore's Army!" he shouted, and there was an answering cheer from the crowd… (36.54)
Neville's good-evil compass never wavers, even when Voldemort tries to tempt him over to the dark side. It would never work – he's a true Gryffindor through and through, and is loyal and courageous to the bone. He's certainly not the second coming of Peter Pettigrew, as we once feared.
"He'll be all right," murmured Ginny.
As Harry looked at her, he lowered his hand absentmindedly and touched the lightning scar on his forehead.
"I know he will."
The scar had not pained Harry for nineteen years. All was well. (Epilogue.43-46)
Good triumphs in the end, and there's no sign of a threat to this happy ending – let's hope it stays that way!