Tell me, Professor...have you ever played Morrowind?
Harry hadn't played the game himself until after he already knew the trick, so he hadn't gotten to figure it out for himself, but he knew how it worked.
There was a skill you could use, called Alchemy. Your character could learn to brew potions with magical effects, and the power of these effects was determined (as it would be in any self-respecting magic system) by your character's Intelligence attribute.
But one of the types of potions you could brew was a Fortify Attribute potion, which would temporarily raise one of your attributes.
Like, say, Intelligence.
And then, with your newly increased Intelligence, you could brew a more powerful Fortify Intelligence potion that would boost your intelligence still further.
At first the effect would be small, that was the way exponential progressions worked. But it would get faster, and faster. If each step increased your intelligence by 10%, after ten steps your Intelligence would be two-and-a-half times what it had been before. After twenty steps it would be more than six times what it had been before. After one hundred steps it would be more than thirteen thousand times what it had been before.
So part of Harry's mind had always been on the lookout for a way to replicate this using the magic that he had found actually existed.
And for so long, he hadn't seen it. For the problem with deontological constraints is that if you never genuinely consider the arguments for violating them, you never truly know whether you should. And Harry had dutifully recited the Rules of Transfiguration in each of Professor McGonnagal's classes, and chastised himself for his reckless experimentation, and stepped carefully around all the useful parts of Transfiguration without thinking about it.
Until Professor Quirrell taught him that lesson.
Holding the Philosopher's Stone would allow you to perform human Transfiguration, would let you safely Transfigure yourself without dying of Transfiguration sickness.
But there were other times you could perform human Transfiguration as well, and that was what Professor Quirrell had done.
If you were confident that you would obtain the Philosopher's Stone in the next few hours...then you could use Transfiguration to your heart's content, Transfigure yourself or anyone else as you wished, and then once you obtained the Philosopher's Stone use it to prevent their death.
It was what Professor Quirrell had done to his student hostages.
And another time you could use human Transfiguration on yourself was if you were going to die anyway, or if you were willing to die for something, and you could accept that you would die when the Transfiguration wore off.
Slytherin's Monster had taught him a way you could Transfigure yourself to enhance the power of your muscles, pouring the power of your magic into them to enhance your body...
But his magic was a part of him in the same way his muscles were, there was something in his body that dictated his magic, dictated how powerful it would be and what effects he could achieve, and he could approach that with the same mechanic. It had taken him most of his week, with the reluctant assistance of Slytherin's Monster and a bunch of rats that had been subjected to what might have been the least ethical series of experiments ever performed, but he had learned how to do it.
And once he knew what to do, knew how to Transfigure himself to increase his magical power, he could do it again, and again, and again.
If there were no limits to the power of your magic, what would you do, if you could do anything?
But this power would not last. You couldn't maintain a Transfiguration on something as large as a human indefinitely, however powerful you were. If the Dark Lord accepted his offer, one condition he'd like to add to it would be access to a Philosopher's Stone, to keep Harry from dying in the next few hours. (Hufflepuff was asking him if he really thought he deserved to live given what he was about to do. Gryffindor was asking if it was worth the effort of negotiating for that, if it would be better to sacrifice himself in order to not reveal that weakness to Professor Quirrell.)
And if the Dark Lord didn't accept his offer...well, then Harry dying would be the least of his worries.
"My magical power iss esssentially without limit. Will not tell you how, am not willing to rissk that you could duplicate the effect. However, I am willing to demonsstrate ass you require. Inflamare."
The ground around them was fire, the air around them was fire, everything turned to red in that torrent of flame...
A wall of water descending, a vast cloud of steam rising.
No more water, no more steam, no more fire, just the Boy-who-Lived and the Dark Lord standing across from one another.
"And how do you think this will kill me? I have horcruxess, many of them, sspread acrosss the world! You cannot find me, cannot kill me! Thiss is meaninglesss!"
"Do not need to find you."
Harry felt sick. This felt too wrong, this was against everything he knew, everything he believed in, everything he wanted, but there was no other way he had been able to find.
"With Aguamenti I can create great quantitiess of water, far beyond what you just ssaw if necesssary. With Arresto Momentum I can move it as I wish, direct it into the Ssun."
"You sseek to put the ssun out?" Harry didn't know why, but the voice sounded, somehow, almost afraid.
"No, that's not what will happen." Harry had read a blog post about it when he was young, about what would happen if you pointed a giant hose at the Sun. "More water will make Ssun hotter, not colder. Hotter, and hotter, and hotter, until eventually it will explode. Ssupernova, it iss called."
Those eyes looked different now, somehow, looked like they had fear in them. Harry pressed onwards.
"Such amazing things we Muggles have learned. How stars are born, how they are preserved from death, how they die. And it had never even occurred to me to consider that such knowledge might be dangerous. Thank you for that, Professor."
"Not enough, boy! You know that I have horcruxed sspacecraft, that..."
Harry shook his head. "Heard once of a way to calculate how much energy is releassed by a ssupernova. Take any number you pleasse, and then make it bigger. It will reach out far enough to vaporize the Pioneer probe and all other human-launched spacecraft. I do not know if that will be enough - it iss mundane force still, iss posssible it cannot affect magical items - but you have taught me ssomething that will."
Harry raised the wand, traced a rune in the air.
"Thank you alsso for sshowing me that, Professsor. With a Penssieve it iss posssible to review memoriess, watch over and over again until I have learned what I musst. Now, even if Horcruxesss sshould ssurvive, I can reach them all eassily with my power, desstroy with Fiendfyre until nothing iss left of you."
The Dark Lord's face seemed almost panicked...and then, suddenly, it was cold again, controlled if not calm.
"Well done, boy. You have gained power sufficient to kill me indeed...but what of your world?" The hand came up, gestured around. "You claim you would reduce all of this to fire, just to kill me?"
"Only if you do not meet my demandss."
"No. That is ssimply untrue, boy. I would believe that you had found a way to lie in Parsseltongue before I believed you would do that. A fine bluff, perhapss, but nothing more. If you wissh to claim to me that there iss ssomething of you that I do not know, if there iss ssome reasson that you would act in complete disscord with everything I have ever sseen of you, then convince me of it now."
And this was a very good point. It was one that Harry's mind had been fighting over for much of this past week. But then he had discovered something that had brought him, if not peace, then at least a kind of grim resolve. Harry raised his wand in a trembling hand and spoke, pouring in power from his limitless reserves, to cast the spell more powerfully than its creator had ever imagined it could be used...
Above them, in the sky, like the sparkling lights of stars only somehow warmer and brighter, little points of red, glowing faint against the bright sky of daytime.
A whole galaxy, a whole universe, worlds beyond count, with light and beauty and happiness of their own.
And to unleash Lord Voldemort on them could not be allowed to happen.
Harry had spoken to people, not many were willing to talk about it but he'd found a few who were, and he'd found out more than he wanted to know about the Cruciatus Curse.
And he had realized something.
He wasn't afraid enough of it.
Because it didn't look scary. You pointed a wand at someone and said 'Crucio', and they screamed. The Emperor had done something worse-looking to Luke in Star Wars, which Harry had watched when he was six and which hadn't particularly bothered him.
If you imagined having the Cruciatus Curse cast on yourself, versus imagining having yourself strapped down and your fingernails slowly ripped out one by one with a pair of pliers, the second one felt much scarier. It felt terrifying and visceral and real in a way that the simple movement of a wand did not.
But there was a reason Lord Voldemort had used the Cruciatus curse to torture people with, to threaten people with, to threaten their families with.
It was because it was worse.
And to unleash this Dark Lord, who thought in terms of pain and sorrow and misery, who would take any action to achieve his ends, not with hatred for others but with simple disregard that managed to be infinitely worse, who thought that an attachment to another person was simply an invitation for them to be tortured as leverage over you, who wanted to tear apart the human race and remake it in his image...
To unleash that upon this flawed, weak, pathetic, broken, beautiful, bright universe was something Harry simply would not allow to happen.
And if that meant the end for him, if that meant the end for everyone he had ever known, if that meant a flash of light that wiped a planet from the galaxy as if it had never been...
He would pay that price.
It was a terrible thing to say, he knew. A terrible thing to do, a terrible thing even to risk.
But when you considered a universe where the Dark Lord rode out from Earth at the head of an army of monsters forged in his image, with the full power of Magic unlocked and ready to heed his call, with a cold darkness that would take everything beautiful in this universe and tear it apart towards his will to power, and then you considered a universe where a single star blazed bright in the heavens and then went out...
There really wasn't any contest.
The Boy-who-Lived faced the Dark Lord, and around them the world spun, one Earth amidst the stars, one Sun shining bright in the darkness, two Riddles standing beneath it, one with eyes of red who would save the world to destroy the universe, and one with eyes of green who would destroy the world to save the universe.
For it is the most common failing of moralists, that they assume the world is fair and good and nice, that they assume there must be a good option as opposed to two bad ones, that they think if you simply don't do anything wrong everything will turn out all right. And sometimes, when the world is warm and bright and man lives in luxury wrought by his hands and his mind, that might even be true.
But when the world is dark and cold, and man is huddled tight in a cave while the beasts roar outside, the only thing he has that can drive back the dark is fire.