When you're under a ton of boulders in the worst pain imaginable, after you became so certain that you were about to die that you gave one of your own eyes to a complete bastard who may or may not have redeemed himself in the end, is the worst time to discover that the strange changes you've been going through lately aren't puberty, and that you might actually survive long enough to get out of the mess that you're currently trapped in despite the pain. You don't know exactly how you're alive, but you have a strong suspicion that it has something to do with the three days that went missing after you agreed to help Orochimaru with one of his numerous experiments.
As you drag and claw your way back to the surface inch by tortuous inch, the hate builds up inside of you. Hatred for Kakashi who is both a comrade you loved and a rival you hated is easy to find, as is hatred for the rest of your clan who constantly rode you and belittled you for falling short of an ever-shifting mark, and criticized you non-stop for failings real and imagined. Hate for Sensei who left you behind to die without once bothering to verify that you are gone, sensei who had always loved Kakashi best, follows soon after. After that comes the hatred for Konoha, Konoha that took your last shred of innocence, Konoha that had demanded the last drop of your blood and your very life and gave you very little in return, Konoha that had gone to war which got you in this mess in the first place.
The hate is what lets you survive as you slowly and agonizingly make your way up towards that faint bit of light you can see above you. The pain would be completely unbearable otherwise. You've blacked out any number of times, and once or twice you may have died only to return to the pain and the climb. Each time you've died, He was there sitting by the fire waiting for you.
You know him, you've heard his story a million times before just as every Uchiha child does growing up. When you were little, you used to be afraid of him, afraid that he would come into your room while you were sleeping and steal your eyes just as he did his brother's.
You've given one of your eyes away already, so it doesn't really matter now does it?
The story he tells you as you sit beside the campfire with him in those brief periods of respite from the pain, periods you wish more and more would be permanent the longer the pain continues, is different from the bedtime stories your parents told you. As you listen to him who may or may not be real, you find you understand him, and understand his hatred of the Senju amongst others including his clan who threw him out for trying to do the right thing.
As you listen to his plans, and the goals he left behind at his passing, you find yourself claiming them as your own. They are a much better reason to live than the one you had previously before your team left you behind to suffer in this place.
After all, what has the Will of Fire gotten you aside from a premature grave under several tons of boulders? You sure as hell weren't appreciated for trying to live up to that ideal before now.
Eventually, after what seems to be an eternity in the darkness, you finally make your way back to the surface where, through some strange miracle, your body regenerates. Despite the fact that you shouldn't be, aside from the missing eye, you are whole.
As you wander for weeks, possibly months, trying to decide whether or not you want to return to a home that doesn't appreciate you, a home that believes that you are dead, you find signs of the seeds of The Plan that Madara had planted long-ago, seeds that were to be carefully tended to by a custodian who has since grown old and died himself.
Half fearing that you are mad, that you've been driven mad by your near-death, you take those carefully tended seeds for your own in order to see that they grow. You claim them for yourself by claiming yourself to be Uchiha Madara who had laid the framework and foundations for your plans long before you were born.
Despite the fact that your heart hammers in your chest each time you go through this ruse as you gather up the scattered elements of Madara's grand plan, hiding behind a mask that covers the fact that you have not yet reached adulthood, and you spend every second fearing that you would be unmasked for the pale imitation that you really are, nobody calls you on your charade, nobody even questions you. Uchiha Madara is a semi-mythological figure to whom subsequent generations have ascribed any number of feats. It is not so hard for people, even shinobi who have been trained to weed out deception, to believe that the man who had controlled the Kyuubi, the man whose titanic struggle with the Shodai Hokage formed the Valley of the End could have somehow continued to survive to this day and age working behind the scenes in order to control the fates of nations.
As you travel, you see the destruction your village has wrought over the decades since it had been founded, the lives that have been ruined by the people you had called comrade, the people who had left you for dead in the pain and darkness that you had clawed your way out of by yourself. You, who had once been kind and hurt when you saw others hurting, felt the hatred you had felt for your village as you crawled from beneath the rocks that were supposed to be your grave, a grave that was oh so far away from anywhere you called home, flare up and grow white-hot.
It is when you meet Nagato who is the result of a generations-long experiment in back-breeding in order to regain the lost Rinnegan that your resolve is finally firmed. Nagato and his friends had been hurt by Konoha time and time again. First, losing their parents in the war, then losing your former sensei's sensei who'd stopped to raise them for a while when the bastard decided to abandon them, not once returning to help, even after they had gotten in over their heads and found themselves in direct competition with Hanzo and his forces when they had tried to make their home a better place.
By the time the war ends, you find that one of your top goals is to destroy Konoha, to break it and the rest of the world in order to rebuild it into something better, something that would have actually been worth dying for. You know full well, that sacrifices would have to be made, hundreds, thousands, possibly even millions of sacrifices, but you can't make an omelette without breaking a few eggs, and when the world is finally at peace, it would be worth it all.
If you hadn't seen evidence that you had somehow met Madara during your travels, you would have believed that you got the information you knew about the Kyuubi from an old and forgotten scroll somewhere. Back when you had the time, you had enjoyed spending your afternoons pawing through the dustiest sections of the Clan archives, and rooting out books and scrolls from corners where even the clan archivists who are a cross between clerks and librarians don't even go.
While part of you is hesitant at this, the last chance to turn back before things go too far, before you kill someone you cared for, the rest of you snarls that it's too late already, and that Minato-sensei deserves to lose someone precious to him since Rin has already been lost through his and Kakashi's negligence despite Kakashi's promise to look after her. If you have to go through the pain of losing someone you loved more than life itself, so too would the bastards responsible for that loss.
Still, a feeling of guilt wells up within you when you kill the guards who should have been comrades, and slaughter the Sandaime's wife, and place explosive tags on the newborn infant whose birth you should have attended as a friend rather than an attacker, and release the Kyuubi despite the fact you know it will kill Kushina, brave, brash, bold Kushina who understood you better than anyone else in the village because she was like you in many ways.
You are almost relieved when Minato-sensei defeats you and saves the village. But, it is far too late, and you are already set on the path that will damn you as you do whatever it takes to save the world from itself.