knowingly condemning people to their deaths, that will look REALLY bad for him.  First of all, it will be known that they were dancing in the woods and casting spells (all forbidden), and then it will be known that they were basically killing people in order to get out of trouble for it.  So far, the fancy judges know nothing about the dancing, and Parris holds an upstanding reputation with them.  If the judges discover the dancing, the judges, and the entire town, will turn against Parris and throw him out.

Secondly, Parris has been a key figure in all of the court proceedings that have occurred so far.  He has almost been like another judge, being helpful and questioning people, and giving the judges necessary background information on all of the people.  He has been instrumental in the accusation of many women.  So, if it is discovered that the women are innocent, and that the testifiers have been lying, it will mean that Parris too is guilty by association.  It will mean that he has been condemning innocent women to death also--not a good position to be in, for him.

I hope that those thoughts help to clarify Parris's position a bit; he wants to protect his reputation as a man of god who raises good children, and as a member of a court that has ousted witchcraft.  Mary's testimony will undermine all of that for him.  Good luck!

Mary Warren is primarily motivated by the same factors that motivated so many of the others at that time in Salem, Massachusetts - fear and ignorance.  Mary was afraid of Abby and of being accused of witchcraft herself.  Like the other girls who pointed fingers of accusation at people, Mary was caught up in the frenzy.  When the girls are caught by Rev. Parris in the woods playing with white magic, they are all afraid of what punishments will come to them.  As soon as they start crying "Witch!" and see that they now become victims instead of perpetrators, they all start using that ploy.  Abby is the ring leader in the play, telling the girls in Act 1 that they need to stick to their story and she threatens violence on any girl that goes against her.  Apparently the girls do not doubt Abby's capacity for violence because they heed what she says even though Mary Warren has suggested that they all just admit to dancing and take their whipping for that misdeed rather than lie about what went on.  Later, when Mary brings the poppet to Elizabeth, it seems that she has simply been keeping her hands busy as she sat in court all day - that she had no ulterior motive in making the little doll.  When Mary goes to court in Act 3 to confess that the girls lied about the allegations of witchcraft, Abby turns against her because Mary's claim makes Abby out a liar.  Abby starts to say that Mary is a witch and out of fear, Mary recants and says that John Proctor made her say she and the other girls lied about being tormented by witches.  Mary is very afraid of Abby and what Abby might do to her.  She knows that if Abby accuses her of witchcraft, and the other girls follow her as they are doing, that she will go to prison and possibly be hanged.  The ignorance comes to play because the people did believe that witches could be among them and that hanging those accused by girls was the right action to take.

John has promised to come to the courts and "bring [Elizabeth] home."  He feels incredibly guilty that she was arrested in the first place; he blames himself because it was Abby's affections for him-spawned from their affair-that led to his wife's accusation and arrest.  He realizes with full force just how far Abby will go, and that his wife was right:  "[Abby] wants me dead, John."  So, he has a very personal sense of vengeance, and justice fueling his desire to get his wife cleared of any charges.  And, it just so happens that Mary Warren, who has been with Abby from the beginning, knows just how fake all of the accusations of witchcraft are.  Mary was there in the forest, she knows why they started crying people out (so that they wouldn't get in trouble for the dancing), and John needs her to testify to this in court.  If he can somehow prove that Abby is being false, it will nullify Abby's accusation against Elizabeth, and Elizabeth will be set free.

However, when John goes to Mary and insists that she testify against Abigail and the other girls, Mary is terrified and refuses, insisting, "They'll turn on me...I cannot do it!"  What she means by this is that she is afraid that if she goes in to the courts and states that they were all faking it all along, the girls will start pretending that Mary Warren is a witch, and unite against her, and she will end up being on the stand, in jail, and hanged for witchcraft.  She knows what power Abby and the girls hold, so she is terrified that she will be their next victim, especially if she testifies against them.  And, she is right.  Act three will reveal just how right she is.