Abigail had been having an affair with John Proctor. During this time, she was also employed in the Proctor household as a servant girl. John breaks off the affair and when his wife, Elizabeth, learns of the affair, Abigail is dismissed (fired) from their home. Even after this, Abigail tries to rekindle the affair with John. He refuses her.
Note that Abigail's parents had been brutally killed by Indians in their sleep. She was then raised by Parris who showed her no real parental affection. As a result, Abigail is one who has always sought affection. When she finally receives it from John, she is satisfied. When he takes that away, she is left alone again, sans the loyalty she has from her friends. While Abigail's actions (accusations of witchcraft, lying, etc.) can not be condoned, we can point out reasons as to why she came to do these things. John led her on. To abandon a young, vulnerable girl who has a history of being abandoned was a huge mistake on his part. But Abigail takes her revenge on John and Elizabeth (and other members of Salem) entirely too far. Abigail and the other girls who accuse people of witchcraft are also the victims of a strict, religious society. They accuse others in order to save themselves when their only real "sin" was dancing in the woods and pretending to speak with the dead (at Mrs. Putnam's request).