Disclaimer: Don't own it – Jo Rowling does.
Not that it really needs to be said: this is very much non-canonical, and isn't even attempting it. It's merely justification, and it's more of a scene than a story, at that.
In the aftermath of revelation:
Dumbledore tried to defuse the younger wizard. "Harry, please…"
Harry interrupted, his patience snapped.
"No! No more! You had me raised as a martyr. You dropped me on the doorstep of a couple that despised me and my family, at a time when my parents' bodies hadn't yet been buried. You ensured that I enter into Gryffindor, forced me to confront Voldemort 3 times, and allowed me to be denigrated and hated for an entire year. Now you tell me that I am prophesied to face Voldemort, but more than that, that I must lose?! How can you do this and live with yourself afterward?!"
Dumbledore sighed, his own patience fracturing. "It was all for the greater good."
"The greater good?! You sacrificed my life and childhood to a dream?!" Harry exploded.
Dumbledore slammed his hand on the desk, ignoring the metal shard of what was once some trinket embedding itself in his fist.
"Yes, Mr. Potter, I did, and I would again. I don't expect you to understand, or indeed to forgive me, but it is and was the right thing to do."
"The right thing to do? The right thing to do would be to get off your wrinkled arse, take control of the ministry, and get the unspeakables involved in circumventing this prophecy, not to mention getting Voldemort's soul out of my bloody head!"
Harry's expression turned cold.
"You're a coward, Headmaster. This conversation is over." Harry turned to leave, shaking with rage, but the door slammed shut in front of him. He turned back, snarling, a bludgeoning curse on his lips.
In front of him Dumbledore stood, an umbra of magic writhing blue-red around him.
"Do you think I haven't tried?!" Dumbledore roared, truly angered for the first time. "You think I was idle while you spent 10 years under the roof of the Dursleys?! You think I haven't exhausted every resource, interrogated every source, called in every bit of political capital I own, to try to find a solution other than your death? Tell me, Harry Potter. Tell me your plan. Tell me your solution to this problem, the extant madman whose very existence threatens not just our world but the entirety of Britain. Tell me. Would you rule over the other wizards, crushing everyone under the iron fist of justice? Would you leave the marked child alone, knowing that in doing so you were consigning the world to death?"
Dumbledore took a trembling breath, and his aura grew wild.
"Why would I want the solution to be your willing death? Why would I want a martyr if I could have anything else? Why would I do what I have done if I had any other solution?"
Dumbledore paused, and the wreath of power withdrew into him.
"Is it right to sacrifice a child for the lives of everyone else? It must be. Perhaps they do not deserve life. You certainly do not deserve to die. But am I to be the judge of that? Am I to decide that a people I know not, their children, and their children's children, unto the final generation, should be put to the sword? In turning a child who should have died into one who must, I have made certain that an untold number of other children do not have to."
Harry had let go of his spell by this point, but kept his eyes narrowed.
"And yet you never intervened. You could have fought Voldemort to a standstill, much more his death eaters, and you allow them life. Why do they deserve to live if it means the eventual death of their victims?"
Dumbledore smiled, though it was strained.
"You think that I am blind to suffering; that my idealism has lead to ruin in places where a more ruthless approach would have paid dividends. I see it. I see that you believe what you do, and I accept it. But, I ask, do you think I am unintelligent? Insane? I follow my moral code, yes, but I do so because it works. Do you know the name of Albert Timmons? Dorothy Wealten? James Brooks? All of them are Order of Merlin recipients for their work in the first war, and all of them once counted themselves among Voldemort's followers. John Anders was among Voldemort's inner circle, and his defection saved hundreds."
The headmaster took a deep breath, but continued.
"Why is it, do you think, that the muggle world was so unaffected by the first war? Voldemort's followers virulently hated those they considered lesser, so why were casualties so few? It is because of how we conducted ourselves, sacrificing valuable intelligence operatives for the sake of preventing largescale terror. Samuel Greengrass – the uncle of Daphne in your year – died to bring us news of an upcoming attack on Manchester. Do you think we'd have seen such remorse if we had killed anyone who even dared to support Voldemort?"
Dumbledore turned, looking out the window onto Hogwarts grounds.
"Moreover, the fact that we did not escalate meant that the war was confined; I'm sure you're aware of the recently ended Cold War between the East and the West. Imagine if we had responded to Soviet action with violence, and set off an escalation towards nuclear war. Voldemort and his followers did not wish to kill everyone, but rather, to rule. They limited their arsenal to deadly but contained spells, keeping a qualitative advantage over our non-lethal armament. Imagine if we had responded in kind: you have seen the damage magic can inflict, I am certain. To maintain an edge over our now lethal response, the Death Eaters would begin using far less controlled – and consequently more lethal – spells. In fact, the vast majority of non-Auror casualties occurred only after the ministry authorized the use of the three unforgivable spells."
He brought his gaze back to Harry, sporting a tired smile.
"I wish I could say that I always follow my conscience, and that any problems with this war are attributable to me alone, but that is simply not the case. You talk of destroying the enemy, but have you considered what might happen after the war, when those who sympathized with Voldemort, who make up a dreadfully large portion of the populace, see the death of friends? Do you wish for a rebellion, a civil war among the men and women of the British wizarding world? Because that is what you will gain if you kill everyone involved. You will have won the war, but lost anything worth fighting for in the process. You would have set the government back decades at the least, and bred a world where vile hatred of muggleborn was now entwined with martyrdom. You would have, to cite a muggle saying, won the war, but lost the peace."
Dumbledore raised an arm and flicked his wand. A glass of water floated over to the headmaster, who took a sip, set the glass down, and continued.
"I have told very few people this, but there was a time, in my youth, where I had similar ideals as Tom Riddle possessed. I, and a good friend of mine, desired dominion over the muggle world, considering ourselves their betters. It took the loss of someone very dear to me to understand what I am trying to convey to you: conflict, especially unnecessary conflict, can only cause suffering. Every enemy you make, every crusade you exhort, every man disillusioned with your cause… they will pay you back a thousandfold. I do not doubt that we would win a total war against Voldemort's followers: for all that they have power, I have connections with the wizarding world around the globe. We could end this war with sheer brutality, calling upon foreign legions to destroy utterly our government and those within it. But that would breed resentment, and hate, and lead to a hundred lesser Voldemorts in place of the one."
Harry dropped his suspicion, but not his accusatory inquisitiveness.
"These are hypotheticals, headmaster. Do your best guesses really take precedence over the lives currently being ruined? Does 'this might end poorly' really justify letting people suffer right now?"
Dumbledore gave a quiet chuckle.
"A Gryffindor to the last, I see."
Harry gave Albus a reproachful look, to which the headmaster shallowly nodded and ceased his laughter.
"Harry, you misunderstand: these are not hypothetical statements. I speak from experience, most notably the end of the wizarding war fought in concert with the muggle Great War."
"Grindelwald's rebellion, you mean?"
The headmaster shook his head.
"No, I mean the Great War. Fought against the backdrop of the muggle World War I, it was a far less deadly affair. The Russian wizards had already gone to their palaces of rime by that point, and both the Ottoman and Austrian wizards remained neutral. The war was fought between our coalition of the Italians, Iberians, British, and North Africans against the Prussian-French."
Harry crinkled his nose.
"If there was a war that preceded Grindelwald's ascension, why is it never talked about? This sounds like something I should have been taught."
Dumbledore chuckled.
"I daresay Professor Binns might have done a fine job of it, seeing as he fought in the very war we speak of, but Cuthbert's teaching style is a matter for another time. To compress the tale, I will say that our coalition invoked a series of magical treaties binding the Americans and Russians to us, and I believe it is best stated that we stomped the enemy forces rapidly...and brutally."
He took a deep breath, and frowned.
"The problem with magical surrender is that it does not take all that much in the way of vindictiveness to commit the losing side to dehumanization. In our haste to punish the Germans and their allies for their acts, a few clauses mandating servitude and execution were included in the binding document. The magical populace of France found themselves enslaved, forced to work in conjunction with their muggle government, and the Prussians impoverished and indiscriminately culled. Grindelwald fed on the resentment, building a base of the disaffected and hateful, and when he finally launched his war, it would not be wrong to say that we deserved it."
Harry crinkled his nose.
"I didn't have the best muggle education – something for which you, Headmaster, deserve a lot of blame – but this sounds remarkably similar to the start of World War II."
Albus released a tired smile.
"The only true difference between muggles and wizards, I find, is magic. We are all human, and that indeed is all we shall ever be."
His expression shifted slightly, chilling to moroseness.
"It is when we forget this that our demons come home to roost. Tom Riddle fears death, Harry, and it is that fear which has driven him to the depths of evil. In fearing death, he has forsaken life, dedicating all of himself to staving off eternity. Were we muggle philosophers, I could see justification for his acts, grand and awful though they might be. But we know that there exists a realm beyond death, that oblivion is not awaits those of us who fall, in frailty or battle both. The great tragedy of Lord Voldemort is that his desire for control has led him to grapple with the uncontrollable, that his entire being is devoted to understanding that which no one can know."
A soft shadow flickered across the aged headmaster's face.
"To seek the impossible is grand, but to be consumed by it...that is something altogether different. In his fear, Tom has lost hope, that most precious of our beliefs. Tom Riddle cannot bring himself—"
"Hope? I'm going to die, headmaster. Voldemort isn't the only one without hope."
Harry bit out the interjection with a snarl, his eyes misting.
"I can see why I must die, but do not preach to me of hope, or of the good of all. Spare me that, at least."
Albus opened his mouth, but no words escaped his lips.
"Save it. I'm going to see my friends."
Harry turned to leave, and this time, the door opened before him. At his desk, Albus closed his eyes, and dreamed that Harry might question why his death must be willing.