Survival vs. Principles

Katniss feels ashamed when Peeta tells her, “I want to die as myself…I don't want them to change me in there. Turn me into some kind of monster that I'm not.” (p. 141) She had been worrying about survival, not principles. Do Katniss and Peeta become monstrous during the games? How do they reach the point of making their stand at the end of the games, when they stake their survival on a principle?