But because the Master is a part of the Doctor -- the Faith part that takes a Neitzschean pleasure in its power -- he can't ever die completely. No second chances, no return to egg-form for another try. No second chances. The Master is just another Reinette, in a way -- somebody who's walked the Doctor so completely that the edges go fuzzy: "We are the same." I'd say that, beyond even the strength of sacrifice, the Doctor's knowing that this balance is forever going to be his problem is a truer expression of power; I also think that it's Martha who saved him from the Master's road, after Rose broke his dependent, human heart. Thinking you can really kill the Master? That's just a little boy, shooting at straw men, tears running down. The Doctor will always be scary, and he'll always need somebody to stop him, because that's how the Master happens: it's a natural consequence of power. That's "42" happening, forty-two minutes at a time, for nine hundred years and counting; ask the poor Family of Blood, or even the Weeping Angels, in their Schrödinger hell. It's the humanity in the Doctor, which Martha and all the Companions have forged and will continue to forge, that keep him on that path, for the most part. Life is balance and it's a constant working: grace is a wave that never breaks. They seem like two great faces, or a very large vase, but in fact they're just lines on paper, drawn in black and white, played on a stage by very tiny, very sexy men. It looks like this:
But it's really just two dots and a curve. It was you who made it real. The Doctor and the Master in conflict was the two sides of a God, fighting for a position from which to perceive his relationship to humanity. The Doctor, who doctors, and the Master, who masters. The open hand and the closed fist. Mercy and Judgment. Either the Last of the Timelords is so in charge of humanity that he can put it into a paradoxical self-destructive loop, like toys, for his own enjoyment and mad quest for war...or he can sacrifice his power, authority, and pride, and let humanity save him. It's the smart kind of faith again, where stories and songs bring you strength from strange places; it's the kind where you realize you can save yourself.
Imperfect execution, I'll admit, but the elements are there. That last is why I don't have a problem with the Messiah bullshit, or the glowing Tinkerbell Jesusiness of it all: it's not about the Doctor becoming super-powerful and not learning his lesson, but about the moment just before that, about that forgotten year, when he realized that there was nothing to be lost in handing humanity the reins. He knows that grace operates for him too, and that, in jumping, he has a chance to save everybody, even the Master. Because the Master is not a foe in the classic sense: he's a brother, someone with whom the Doctor's relationship verges on the romantic, and a necessary element of both the universe and the Doctor himself. When the Face of Boe says, "You are not alone," it could be a wish: the Master is just the plutonium inside.