Wilf shrugs. "1948, I was over there," he points. "End of the Mandate in Palestine. Private Mott. Skinny little idiot, I was. Stood on this rooftop, the middle of a skirmish... It was like a blizzard, all them bullets in the air. The world gone mad." All that chaos down below, just like Adelaide. "Yeah, you don't want to listen to an old man's tales, do you?" The Doctor smiles sweetly at him finally, reminding him he's 906.
"We must look like insects to you," Wilf says, finally grasping a little bit of it, but the Doctor just stares at him lovingly. "I think you look like giants."
Wilf takes out the gun.
"Listen, I... I want you to have this. I've kept it all this time, and I thought..." The Doctor won't take it, of course. Wilf begs him, but he can't. "You had that gun in the mansion. You could have shot the Master there and then." Wilf shrugs, quietly. Eloquently. "Too scared, I suppose."
The Doctor watches him.
"I'd be proud."
"If you were my dad."
The tears spring up in his beautiful old eyes. "Oh, come on, don't start."
Wilf's namesake was a soldier, and a poet. Unafraid of love as he was of war. He was very beautiful, and died too young. He wrote once, "Above all I am not concerned with Poetry. My subject is War, and the pity of War. The Poetry is in the pity."
But then, if the prophecy said he's going to knock four times, that's obviously the knocking in the Master's head, so the Master is going to kill the Doctor. The Doctor nods. Then, Wilf reasons, think of his Time Lady, he just needs to kill the Master first.
"And that's how the Master started," the Doctor says. Wilf feels bad, even though he didn't say it as chasteningly as he would've anybody else. "It's not like I'm an innocent. I've taken lives. And I got worse: I got clever. Manipulated people into taking their own." He is ashamed. He goes darker than the Vinvocci ship. "Sometimes I think a Time Lord lives too long."
Wilf looks at him, the hollow fear of him, and holds it out again. He'll deny you three times, that's how this works. Wilf's canny. "If the Master dies, what happens to all the people?" The Doctor pauses too long, before he lies that he doesn't know. "Doctor, what happens?" The template snaps, and everybody lives, and everybody goes back to normal. "Then don't you dare, sir. Don't you dare put him before them."
That's what he says, but not what he means. What he means is we're in dead man's alley, in the Age of Steel, and in that world you can't stay clean. You can try, you can hold to your ideals until the rats eat your bones. What he means is that the Doctor can't put himself, his godlike innocence, above them all. Can't save Ianto when it means a crimson tithe. Can't shove that hardness off into Pete's World, like Ten Five, for being too ugly. Can't afford to stay above the fray, soft and uninvolved. You can't put those ideals before the innocent, without losing them altogether.