The Master's problem isn't madness, not even the madness of war: it's his belief that there's a fundamental and insurmountable difference between him and anything else. The Master believes that he's alone. He is mistaken.
FIVE: A Wedding
Everything that means anything is in the darkest and most neglected corners of your house, in the places you're not sure of. In the bug rooms and the cobweb corners, those are the things that keep you from being free. But the truth is that those things belong to you, helping to make up what you are, and by ignoring them, you give them so much power that you could have for yourself, as glory. What's horrible and wonderful depends a lot on your perspective. It depends on how big your house is.
Now it's the Doctor strapped down, with the Master whispering into his ear. He calls out to himselves, all 6,727,949,338 of him across the world. Still with no clue as to the real danger; to the untempered schism that drives him. The United Nations, UNIT, the Central Military Commission in Beijing. All the armies and all the strength; the Master winks lasciviously at Wilf as they count the soldiers. The Earth has become a war machine. "Nothing to say, Doctor? What's that? Pardon?" He leans into him, delighted, speaking right into the Doctor's taped-shut mouth: "Sorry?"
Wilf jerks angrily in his bonds: "You let him go, you swine!" The Master laughs. "Oh, your dad's still kicking up a fuss." Wilf says he'd be proud to be the Doctor's father, offended, and the Master shushes him jealously. "Listen to your Master." He turns back to the Doctor, but Wilf's phone chooses that moment to ring: it's Donna, terrified, surrounded by a man she can't remember. Behind every door is a face you haven't met yet. The Master grabs the phone and hears her voice; wonders why she didn't change. Wilf duly explains about the Metacrisis, and the Master campily recoils. "He loves playing with Earth girls. Ugh!" He sends himselves after her, corralling them instantly to catch her. Wilf screams for her to run, but she's caught. And just before they catch her, she screams and burns, cutting loose with memory and rage, and drops the Masters all around her. She falls too, even as the Doctor's winking cheeky at the Master. "Did you think I'd leave my best friend without a defense mechanism?" He promises Wilf she'll be okay, and the Master starts in about the TARDIS.