Certainly, for me, the moment that provokes the strongest emotional response in Act Three is when Mary Warren turns on John Proctor, once again taking sides with Abigail and the rest of the girls, and accusing him of being in league with the Devil.
The reason it affects me so deeply is that Mary Warren knows, absolutely, that the girls are lying, and thus committing murder; and, ultimately, she makes the same choice. She has been brought to the court, by Proctor, for the sole purpose of exposing the girls' lies. Yet, when she feels herself to be in danger, when the girls are starting to accuse her of sending out her spirit to attack them, she turns on the truth and her employer, knowing full-well that her testimony against him will likely lead to his conviction and death. It is a truly horrific moment. Before she runs into Abigail's arms, she says -- lying, and knowing that she is lying -- that
"[Proctor] wake me every night, his eyes were like coals and his fingers claw my neck, and I sign, I sign . . . [....]. No, I love God; I go your way no more. I love God, I bless God."
She fabricates a story of how Proctor forced her to sign the Devil's book, in order to explain why she accused the other girls of lying. She says, in other words, that the Devil forced her to lie then, and now she tells the truth. In reality, it seems more likely that the Devil coerces her to lie now because it is the easier thing to do. To see her lie, so callously, taking another's life into her own hands because she fears losing her own, is abominable in every moral sense.