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From this end of things, ten regenerations and almost nine hundred years later, it seemed almost impossible that things had once been so simple. It was hard to believe that he'd once just been Theta, growing up with Koschei always by his side. They were always inseparable, from the moment they met. Theta had been the first person poor Koschei had told about the drumming, and Theta hadn't slept properly for a week due to his concern.
They were young, and they were happy, before Koschei went mad, before they grew up and apart. The Doctor couldn't remember a time he hadn't loved the Master, not really. They'd grown into it together, explored each other's bodies and minds as they developed. Always together, always in love, even when they weren't.
They had caused each other so much pain and grief over the years, because once that young love hit the world of the adult it went wrong, somehow. Koschei became jealous at around the same time he became the Master, and Theta's natural restlessness only grew when he became the Doctor. They fought for their love and lost it when the Doctor ran away so many years ago.
They'd had chances since then to make it right, but centuries of frustration and pain clouded the problem. They were grown men with their own places in the world, both utterly convinced they were right. It was love gone horribly wrong as the Master's madness caused him to do horrible things.
He was the Doctor, it was his job to make things better, but he couldn't do it for his friend and lover. In a way, death was a mercy for him, if not a particularly long lasting one. He kept coming back, and they kept missing their chances.
Until that last time, when they'd both finally changed the game. For a moment, one shining moment, they had each other again. Everything would be different... except it wasn't.
He died, again, and this would be the last. He'd chased his demons all the way back to Gallifrey just in time to burn with it. The final irony-in the end, the Doctor had killed the Master after all.
No, the Doctor realized. The Master had sought his own death, to save the man he loved. And that was the finest gift in the world.