If the Master in this story is one-half of a Timelord, then John Simm is the perfect person to place opposite Tennant. From the Martha/Lucy, light/dark Companion stuff, to the childish ADD, he's a match physically and energetically as much as philosophically or psychologically, as a character. It makes me wonder what an Eccleston-era Master could possibly have been like; Severus Snape is my best guess. Saxon even takes on a warped version the burden of Ten's loneliness, most especially in his need for Ten to hear the drums of war. All along, he's been convinced that it's a Timelord thing, and that Ten's just denying him out of some kind of pique. If the drums are his alone, then he's a madman: he is evil for its own sake, destruction uninformed by any directing or rational force. Which may or may not be true, but Saxon's not allowed to think that: evil can't look at itself. However, if Ten can hear them too, they'll be united. The conflict has to be resolved either way, because the point of stories is to resolve conflicts, but if the Master is on top when that happens, we won't have any more stories. And the Doctor's never heard the drums: in this case (alone, perhaps) the Master is a tree without a forest. "It's just you," the Doctor says. The Doctor is the true Timelord, and the drums are just a sign of the Master's illness. And the Doctor weeps for him, and so did you, and don't try to deny it.
But that's not the most beautiful reversal and redemption in this piece. There's another one we're missing ("If this is alchemy there will be three kisses, remember that?"), and it has to do with the believer's responsibility to the divine. Gods thrive on belief, like we all do, in the same way that forests thrive on trees, or text on words, or images on sight. In the Big Three, this belief is part of the same postal network that brings messages and babies from the Lord, strength in times of crisis, and the rest: it's a ladder, stretching down from Heaven and up from Earth, and it's ridden by messengers. Chief among these are the Archangels, who have responsibilities in Creation beyond simply messengering. They form a ladder of their own, and they rush up and down it like functionaries, doing this and that; if angels are voices, archangels are hands, and strong backs. In this story, they bring about the most beautiful reversal of all: where once the heart of the TARDIS took infinity, all the Everything of time and space, and gave it spiritual expression through Rose Tyler, the Archangel Network -- made by humans, operated by humans, tainted by the Master just as the TARDIS is -- takes all the mental power of seven billion souls and gives it physical expression. Gallifrey gave us the TARDIS, and the TARDIS gave us the Bad Wolf; but it's the Good Wolf that saves the Doctor in the end, and that's all manmade. I wonder what he felt, in that moment. If he thought of her, just on the other side of the wall, listening. If he said goodbye.