The worst part, the Doctor thought, wasn't the Toclafane or the torture or the Master torturing his friends in front of him. The worst part was how happy his old friend seemed to be as he did these things. His oldest friend, who he had once played with, learned with, fought with, now laughed gleefully as he gave the order to decimate a tenth of the population of Earth, who clapped his hands together and giggled-giggled-as he tortured the Doctor's friends in front of him. The worst part was the way he whispered, gleefully, in the Doctor's ear, all the things he would do, was doing, to Earth and the wonderful people on it, to the Doctor's friends and loved ones, to the TARDIS. The way the Master would grab him, hold him tight to his chest in a mockery of the intimacy they once shared, and whisper all the things he couldn't think, shouldn't think-
"Watch them burn, Doctor. Just like the Daleks. Just like the Time Lords. Do you remember that, Doctor, when you destroyed the Daleks and the Time Lords, all in one hit? Just another days work for the great Doctor, I suppose. And at the end, you just walked away, no one stopping you, nothing holding you down. Free to wander the universe with your filthy human friends, just like you've always wanted."
And the more insidious things, whispered in the dead of night, which made their way into his head and made him hurt, all over, inside and out.
"Perhaps... Did you enjoy it, Doctor? For a doctor, you always seemed to have a bit of a penchant for causing destruction...One might almost say you enjoyed it. Did you enjoy watching Gallifrey burn? Did it excite you? All of your problems, gone up in smoke, just. Like. That." Followed by that high, mad laugh that made the Doctor's skin crawl, made him want to run away, to jump from the Valiant, to peel off his skin and reveal a different self underneath-just like one of those funny earth dolls, with lots of different copies of him inside, each slightly different.
But even that, even the threats, the manipulation, the torture, wasn't the worst bit. The worst bit was that the Doctor still loved him, still wanted, more than anything, to somehow be back on the planet with the red grass and two moons, watching the sky and planning where they wanted to go. The worst part was that, even with all the terrible things the Master had done, was doing, he still forgave him. Which is worse, in the end? The ruthless dictator, or the 'benevolent' deity that stands aside and watches as others suffer in his place, only to forgive the dictator for inflicting the suffering?