"That's what they were planning in the final days of the War," chokes the Doctor. "I had to stop them."
The end of time means no more trouble. The Age of Steel is cold and unchanging. And without time, without change, everything dies. When nothing changes, everything dies. The fight the Doctor's been up against for five years: Cassandra, the Face of Boe, the fixed point of Jack Harkness. All of it a memory of what the Time Lords made him do. Nothing is more offensive than suicide, not even entropy. But this wouldn't be heat death: It would be cold.
"Then... Take me with you, Lord President. Let me ascend into glory," the Master smiles, and hopes.
"You are diseased," Rassilon says, "Albeit a disease of our own making."
The Master's feelings are hurt; Rassilon raises his glove to kill him, but the Doctor's had enough, and cocks the gun at the President of Gallifrey. "Choose your enemy well. We are many. The Master is but one."
And from the other side, the temptation: "But he's the President," the Master hisses. "Kill him, and Gallifrey could be yours."
The Doctor immediately turns the gun on the Master: That's too gross even to consider. And, as the Master quickly figures out, he's still the link. Kill the Master, and the Time Lords and Gallifrey snap back to the Timelock. "You never would, you coward," the Master says, dying before him. Goading him. Hoping he will. "Go on then. Do it." He stamps his foot madly, but the Doctor can't do it. The Master shakes his head, sadly.
The Doctor swings back around on the President, since he's the other end of the link, and Rassilon nearly laughs in his face. "The final act of your life is murder. But which one of us?"
Impossible choices. He breathes, as Wilf watches, terrified by what he is about to become. The hollowness he invokes. Because there is a way in which the narrative brings itself about. There is a way in which he invokes this. The Master comes to the Doctor when he's called. When the Doctor gets too power-mad, the Master snaps into being. He forced Adelaide into impossible choices, and the madness of the Master brings Gallifrey through. In the Age of Steel there is no grace, no third way, no possible way to move forward without getting your hands dirty. No direction you can jump.
As the TARDIS sings, Wilf's Time Lady lowers her hands, and meets the Doctor's gaze. Her son.
Her eyes well up with light, a weeping angel. Wilf stands behind him, and the Time Lady before. The best humanity can offer, and the best of Gallifrey. The soft world, and the hard. They meet in him. There is always a better way. Nothing in us must die to heal the world.