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He grew up around war. Training and training and training to be a soldier. His life lead up to this moment. The initiation, after which he would join the infinite fight against the Time Lord's oldest enemies. He was young, eight. The elders lead him to the Untempered Schism, a gap in the fabric of reality. He had heard many stories about this experience from others who had already seen it. None forgot this moment. But none of them ever spoke of what they saw either.
He wasn't scared. Not enough to show. His curiosity got the best of him. All of his life he had been alone. He was part of the greatest civilization, which swore never to interfere. He wanted to explore the galaxies and the vastness of the universe. He was one of few, which is why he had spent his childhood alone, because he was different than the rest.
The schism showed the time vortex. A friend he had went mad when he looked into it. The young lonely child looked into the gap and he saw himself, his life. He saw how he was alone now but would grow more so as time passed. He saw that he would fail to live up to his name, the Doctor, and why he had to do it. It was the only way to end the war. The Doctor had to become the exterminator of billions. That wasn’t the only thing that he would have to do. There would be worse horrors. He would lose many, from the war, because of time, because he would be too late, because he didn’t know them yet.
The elders did not know why he ran. He was a soldier. He was trained to fight. They would never understand what he started to fight for once he became brave enough to face his fate.
He never stopped running.
For centuries he would run. But one day he would have to go back for he would be called back to the Time War. By then he had grown up, some, enough to accept and understand what the universe told him that he had to do. He found the moment and used it to lock the war together trapping everyone, his enemies, his friends, his family, everything. He was not just there in the final days of the war. He was the one to end it.
He would spend the rest of his life trying to repair the damage that he had done when he forswore his name. He stayed the Doctor to make up for becoming the Slaughter but it would never be enough because he became clever. He never killed but he manipulated others into doing it themselves while telling himself that he was doing the right thing. The time war would not be the last war that he would have to end. The Doctor was a proper soldier and he would die in battle, but before that could happen he would have to lose everything all over again. He would first become everything, then nothing at all.