Harry stopped believing in good and evil the week his parents, Sirius and Regulus all came back from the dead. Regulus, at least, had never actually died, as it turned out; it was incredible that the younger Black had managed to fool Lord Voldemort, but in retrospect it was also pretty clear that Voldemort hadn't been thinking very clearly during those last days, or he never would have given Lily the opportunity to die for her child.
So Regulus' return wasn't completely absurd, even if it was a total shock that he'd spent all those years living as Stubby Boardman. And as for Lily herself, Harry had suspected from the moment he heard of Inferi that of course Voldemort would raise his parents from their graves to try to use them against him. He'd thought, at least, that they would be disgusting, creepy beings like the Inferi in the cave where he had found Regulus' fake Horcrux with Dumbledore (who did so much talking as his portrait that he might as well not have died).
But Lily and James looked and acted pretty much like normal human beings, except when Voldemort ordered them to commit some bloodthirsty atrocity. Being animated corpses under his command, they obeyed enthusiastically, then went right back to calling Harry "sweetheart" and acting sickly romantic like the teenagers they had practically still been when they died. Harry found it embarrassing to be around them and decided he preferred his fantasy version of his mother, who'd been so devoted to the offspring for whom she gave her life that she didn't spend so much time ogling her husband.
Still, Sirius' return was the real deal-breaker. There hadn't been a body, and Harry was aware that in every Muggle detective story, that would have meant that the presumed deceased would turn up alive and well. Yet Dumbledore had been so firm in assuring him that Sirius was really gone and that nothing could wake the dead...but of course Dumbledore had been wrong about so many things.
The day Sirius came walking through the door at Grimmauld Place, smiling and thumbing his nose at his mother's portrait and grabbing Lupin like he wanted to -- um, embrace him like a brother, Harry didn't feel blissful, he felt cheated. And not only out of his house, though the Ministry ultimately ruled in his favor when Sirius challenged his own will as invalid since he hadn't stayed dead. Evidently, death was meaningless, and what did that say about life?
It was relatively simple to turn James and Lily back to the side of goodness and light. The woman who cared so little for her own existence that she had chosen to die, in what she fully believed would be a useless attempt to save her child, proved to be just as malleable as everyone hoped -- just as malleable as Nymphadora Tonks, in fact, who was persuaded to switch her attentions from Remus to Regulus with just a little flirting and a couple of drops of love potion pilfered from Slughorn's private stock.
As for James, he only had to be reminded that he was a good guy to turn him back into a noble hart. Good guys did not follow orders from Voldemort, even if they did brag about their talents and taunt clumsy students and strut around showing off with the Golden Snitch until their own sons' girlfriends had to prove they were just as skilled at playing Seeker.
Much as Harry admired Ginny for this, he refused to get back together with her upon learning that she had consoled herself in his absence not only with Dean, Neville, Seamus, Ernie, Colin, Terry, Justin, Zacharias and Firenze, but also with Pansy Parkinson. Even though Harry understood that Pansy had been grieving over Draco's absence just as much as Ginny had been grieving over Harry's, he couldn't bear the thought that his girlfriend had gone to bed with a Slytherin.
Ron felt his pain -- it was rather upsetting having a sister known as the "Snitch-Snatching Slut" -- but he was too busy these days trying to impress Hermione with his extremely inferior intellect and only average Quidditch skills to spend much energy worrying about it. And the twins thought it was fantastic that Ginny was so cooperative trying out all their experimental aphrodisiacs. Fortunately, Molly didn't mind that Harry had broken up with her daughter and continued to bake him biscuits.
Anyway, with Sirius, the Potters and Regulus all working for him, it took far less time than expected to track down and destroy all of Voldemort's Horcruxes, though Harry had never in his life experienced anything as painful or disgusting as having the fragment of evil soul embedded in his own head removed by having Moody suck it out his nose. The final battle took place the night of a full moon, so Sirius wasn't there, having gone off with Remus to -- um, embrace him like a brother. But Hagrid had turned up as a one-man cheerleading squad and helped McGonagall and Shacklebolt pick off Death Eaters by hiding them behind his back.
Killing Voldemort ended up being a cinch. Apparently one-seventh of a soul was just not enough juice to defend himself against the gleeful, wanton Avada Kedavra of the Boy Who Lived. The Dark Lord was as hideous as a corpse as he had been in life and Harry wasn't at all sorry when Wormtail set about gnawing off Voldemort's face. Naturally Harry killed Pettigrew too, though not before enjoying this Death Eater feast.
Murdering Snape proved somewhat more difficult, for Harry couldn't help but feel a certain amount of guilt, now that he knew Snape had been on his side all along and had been trying to teach him the things that old fool Dumbledore had insisted he was too sweet and innocent to learn. Plus it was sad that Snape had watched his Muggle father beat his mother to death right before his eyes. But Snape had been inside Harry's head, had witnessed his most humiliating adolescent memories, and might have been the one wizard left alive after the war who was more powerful than Harry himself; so he had to be taken out with a quick and dirty Sectumsempra, which everyone assumed the Half-Blood Prince must have turned on himself in despair after watching Lucius fall like a dark angel protecting his son.
Lucius Malfoy didn't live long enough to know that there had been no need, for Draco, too, had switched sides and was now Harry's lapdog. He was fiercely devoted and very pretty, but Harry often thought he'd preferred Draco when the Slytherin still had some semblance of a spine. How ironic that Sirius had called Snape "Snivellus" when the appellation was so much more appropriate for Draco! With Ginny gone, Harry was pleased to have someone around who'd drop to his knees and show his gratitude on a moment's notice, but to tell the truth, Draco was a bit of a bore in that regard too, and Harry occasionally thought of lifting the Imperius curse long enough for Draco to hit him with a couple of those wicked-good stinging hexes of his.
Scrimgeour lasted only three days in office after the war, replaced by Shacklebolt, whom everyone agreed might make it easier to deal with Muggles since Kingsley had been working in the other minister's office for many months, salvaging the British economy and overhauling the tax code. Like all of the members of the Order of the Phoenix, the new Minister of Magic was perfectly willing to suck up to Harry and give him any token job he wanted -- Auror, Deputy Minister, Headmaster of Hogwarts, Captain of the All-England Quidditch Team -- but having had a taste of Voldemort's power, Harry wasn't terribly interested in any of those positions. His mother hadn't sacrificed her life just so he could grow up to be a professional athlete.
No, Harry Potter knew what Horcruxes were now, and how to bring back the dead, and how to perform curses that Dumbledore himself had never had the chance to use. He wouldn't be wasting his fine Gryffindor education on teaching or chasing petty Dark wizards. He had a perfect home base in the Noble and Most Ancient House of Black, he had minions, he had inherited a fortune, he had powers and protections beyond that of any other wizard. And the anagram of "I Am Harry Potter" was "Hire A Mr To Party" which sounded loads more exciting than "I Am Lord Voldemort."
Mr Harry Potter was going to be the new Dark Lord, and instead of wasting his time hunting down Mudbloods and blowing up bridges, he would walk on water, raise the dead and be revered for all eternity.