"Evil can't look at itself." Evil is confusion of purpose. It's no coincidence that the Doctor defeats the Master when he's at his most powerful: that's him taking on the Master again for his own, by looking at him, by loving him, by letting him out of the box and back into the light.
"You could be so wonderful," the Doctor muses. "You're a genius. You're stone-cold brilliant. You are, I swear, you really are." The Master loves it. "But you could be so much more."
"You could be beautiful."
"With a mind like that? We could travel the stars, it would be my honor. Because you don't need to own the universe: Just see it." The Master stares at him. "To have the privilege of seeing the whole of time and space, that's ownership enough."
The Master allows himself the audacity of hope for a moment, and wonders if the noise would stop. The Doctor promises he can help. He read each wound, each weakness clear... And said -- Thou ailest here, and here.
The truth of war is mere boys, firing on straw men, tears falling from their eyes. When we talk about the drums of war, when we talk about the sound of drums, this is the truth: Nobody ever wanted to be a villain, and nobody ever wanted to kill. They got there by cruel fate, and they must be loved.
"I don't know what I'd be without that noise," The Master says, tired. The clemency of madness.
"Wonder what I'd be," the Doctor says sadly, "Without you."
And for a moment the Master smiles, just a little less alone; thinks of the Gallifrey plains, under the sun. He swallows, and smiles. "Yeah."
FOUR: A Boy
Wilf asks about the drumbeats, and -- still gentled, for a moment -- the Master obliges.
"It began on Gallifrey. As children -- not that you'd call it childhood, more a life of duty -- eight years old. I was taken for initiation, to stare into the Untempered Schism." The Doctor explains how it's a gap, in the fabric of reality: "You can see into the Time Vortex itself."
And it hurts.