Much of the final scene in Act III brings out the sequence of events that leads to Proctor's arrest.  Proctor has brought Mary Warren with him to "fight fire with fire."  He plans on using her to impugn Abigail's credibility in the courtroom setting.  When it becomes clear that Mary cannot stand up to Abby, Proctor takes the next step by confessing his "lechery," his affair with Abigail.  Danforth brings in Elizabeth to confirm Proctor's statement. Unaware that he has already confessed and thus sullied his name, Elizabeth panics and lies in the hopes of maintaining her husband's "good name."  Through continued badgering, Mary breaks down and implicates Proctor as doing the work of the devil.  Proctor receives her accusation with shock, at first.  Then, Proctor turns his anger on the court.  He calls Danforth a form of the "anti- Christ" and then proclaims, "God is dead."  Proctor says this as a reflection of the legal system in Salem, one in which those who are legitimately evil possess power and those who are good are powerless. Proctor's final speech to the court seals his fate and his arrest:

A fire is burning!  I hear the boot of Lucifer, I see his filthy face! And it is my face, and yours, Danforth! For them that quail to bring men out of ignorance, as I have quailed, and as you quail now when you know in all your black hearts that this be fraud - God damns our kind especially, and we will burn, we will burn together! 

From a position of not wanting to get involved, Proctor has moved to a point where he feels bound to speak the truth and no longer pretend to ignore the hypocrisy in Salem. The result is that he is arrested and sentenced to death.