The Impossible Astronaut - S6-E2
Revealing mistake: When the Doctor is struck by the green light emitted from the astronaut, his right thigh momentarily disappears due to the animation. (This is even more visible when looking at the still of this shot on the Doctor Who website). (00:07:06)
Chosen answer: The Master knows that deep down, he deserves death for the crimes that he's committed throughout his life, and since he regards The Doctor as his arch-foe, he expects it to be at his hands. The fact that The Doctor is still willing to forgive him for all of his crimes hurts him more deeply than death would.
Captain Defenestrator