Why does Sarah Good lie about practicing witchcraft in The Crucible?

Readers learn that Sarah Good has confessed to practicing witchcraft and being in league with the Devil when Mary Warren returns home from the courts on the first day of the trials, and she is questioned by her employers, John and Elizabeth Proctor, about the proceedings.  Mary says that she did not intend to accuse Sarah Good because

she sleep[s] in ditches, and [is] so very old and poor.  But then -- she sit there,...

Readers learn that Sarah Good has confessed to practicing witchcraft and being in league with the Devil when Mary Warren returns home from the courts on the first day of the trials, and she is questioned by her employers, John and Elizabeth Proctor, about the proceedings.  Mary says that she did not intend to accuse Sarah Good because

she sleep[s] in ditches, and [is] so very old and poor.  But then -- she sit there, denying and denying, and I feel a misty coldness climbin' up my back, and the skin on my skull begin to creep, and I feel a clamp around my neck and I cannot breathe air; and then -- entranced -- I hear a voice, a screamin' voice, and it were my voice -- and all at once I remembered everything she done to me!

Mary does actually believe that Sarah Good has tried to kill her, but it is more difficult to definitively account for how she came to feel these physical sensations in the court.  Next, Mary told the judges a story about how Sarah Good would come to their door to beg for food, and when Mary turned her away, she would mumble.  Once, Mary became very ill for the next two days.  The judges ask Sarah Good what she was mumbling, and she says she was repeating the Commandments to herself.  When the judges ask her to say the Commandments, she is unable to remember even one.  Once they see that she is lying, the judges call this "hard proof" of her guilt, "hard as rock." 

In seeing that she was about to be convicted of witchcraft, and knowing that a conviction meant certain death, Sarah Good likely decided to save her own life and confess.  As a beggar, she really has nothing to lose by confessing.  So, she invents a story about making "a compact with Lucifer," saying that she "wrote her name in his black book -- with her blood -- and bound herself to torment Christians till God's thrown down -- and we all must worship Hell forevermore."  She tells them what they expect to her, they believe the confession to be authentic, and she maintains her life.