I actually think the turning of the girls against Mary Warren in Act III is one of the few absolutely terrifying and disturbing sections of this play. Abigail, brought into court to answer the charges brought against her by Proctor and Mary, uses her "talent" of being God's instrument and identifying the work of Satan to basically cast aspersions on Mary's character and say that Mary is casting her spirit against her and the girls. What is made more effective is the way that the rest of the girls join in this charade. Note what Abigail says to start this off:
ABIGAIL, looking about in the air, clasping her arms about her as though cold: I--I know not. A wind, a cold wind, has come. Her eyes fall on Mary Warren.
The other girls at this stage begin to join in, and begin shivering and chattering their teeth to sugest that Mary Warren is casting her spirit on them in a malignant way. This of course represents the way in which the girls turn on Mary Warren and prompts John Proctor to reveal his trump card which implicates both himself and Abigail of adultery.