In Act III of The Crucible, John Proctor loses control of his ability to try to please the mob and stops feeding into the hysteria in Salem by refusing to lie about his affair with Abigail Williams.
In Act III, Proctor is forced to testify in defense of his wife, whom Abigail has accused of witchcraft. After a series of gentle pleas from Proctor and a signed document testifying to his wife's good character, Mary Warren (the housemaid), testifies on behalf of Elizabeth. It's then that the girls, led by Abigail, go into hysterics pretending to be possessed by the devil. This is when Proctor loses control and shouts "Whore! Whore!" at Abigail.
As this act continues and Elizabeth, who has not been around throughout the testimony, comes and lies for her husband and says he has not had an affair with Abigail.
At the end of the act, Proctor loses complete control and ends up telling the court, "God is dead!" and then continues with his biggest condemnation of the court: "For them that quail now when you know in all your black hearts that this be fraud—God damns our kind especially..."