He’s been ignoring the sandwich out of pure spite, despite how good it smells, despite the way his empty stomach grumbles. At some point, though, he decides two things: that he can’t stand to continue listening to the Doctor’s demands that he talk about his feelings on an empty stomach, and that if (when) he gets himself kicked out of the TARDIS, there’s no telling when he’ll have the opportunity to eat next.
He finally tears into the sandwich like he’s starving, bread crumbs scattering across his beard. He eats half of it in only a few enormous bites as he gathers his thoughts.
Strategy becomes to be as unpleasant as he can possibly manage. Remind the Doctor of why they can never be on the same side, of why it’s a waste of time and energy to try to help him, in an instant. Self preservation means saving himself the slow torture he endured as Missy; trying to live up to the Doctor’s expectations only to slowly let him down over time. Tear it off like a bandage. He’s irredeemable. He doesn’t claim to be anything else.
“You know why you regenerated, don’t you? I killed you. Not as Missy. Just now. Twenty minutes ago. A few days ago..? How long have I been out? Doesn’t matter. You died because I killed you because I hate you and there’s nothing to discuss. So, if you’d like to put me back on the ground, outside, alone, where you found me, that would be great. I’d appreciate it.”
What kind of shock and horror was the Master expecting? He'd tried to kill him many times. It seemed he'd finally succeeded.
"That's unfortunate. I'd always sort of hoped you didn't really want to, in the end."
The Doctor wasn't keen to let his disappointment seep through his cherry demeanor. That meant losing control of the situation.
"Nonetheless, as much as you hate me, I don't hate you. Not at the moment, at least. So I can't in good conscience let you out into the wild world in that condition, and if it makes you feel any better it's because I'm holding you under arrest, too. I can't in good conscience unleash you on the world, either."
He cupped his ear. "Ah? What's that? You don't like it? Well, too bad. Prisoners aren't supposed to like being held captive. I'm going to be tough on you from here on out. It's my rules or nothing. First thing's first, once you finish eating, we're getting rid of that absolutely dreadful Rasputin beard." He touched his own cheeks, miming a beard as he made a face. "I'm not letting my oldest enemy die looking like that."
