Mary was a very easily swayed character or for lack or better words, spineless. You can assume that she began her role in the witchcraft accusations because she was bored and tired of having no power as a woman and a servant. She begins to feel bad for her part in the whole affair when John Proctor makes her admit that she was lying, however, her character was not strong enough to hold up in the face of being accused herself. This is proven true because When clearly innocent people begin to be convicted, however, Mary feels bad about the whole thing. The first sign we see of Mary's guilty conscience is when she makes a poppet (a doll) for Elizabeth Proctor, who she currently keeps house for. Abigail has brought Elizabeth's name up in court, and Mary knows that Abigail did it only for vengeance. Mary was there when Abigail got Tituba to put a curse on Elizabeth, and she also knows about Abigail's affair with John Proctor.Mary's feeble attempt at recompense backfires terribly, however, as Abigail uses the poppet to frame Elizabeth for witchcraft.
This, of course, makes Mary feel even worse and she agrees to go with John Proctor and testify against Abigail in court. Mary's ultimately spineless nature is revealed in the court scene, when under pressure of being hanged she once again flips, accusing John Proctor of witchcraft and Devil worship.While Mary causes a lot of harm in the play, she lacks Abigail's maliciousness. She's just a weak girl