In act 4 of Arthur Miller's play, The Crucible, a fictionalized dramatization of the Salem witch trials of 1692, Reverend Samuel Parris tells Deputy Governor Danforth, who was presiding over the trials, that Parris's niece, Abigail Williams, "has vanished." Rev. Parris tells Danforth that his daughter, Betty, told him that she heard Abigail and Mercy Lewis talking about ships the week before, and both Abigail and Mercy have now been missing for three days. Rev. Parris adds that just this evening, he also discovered that his "strongbox is broken into," and that the entirety of his life savings is gone, implying that Abigail and Mercy took his money when they left Salem.

Danforth offers to send a search party for Abigail and Mercy, but Rev. Parris says that they are probably already aboard a ship and therefore are unlikely to be found.

Rev. Parris offers a possible explanation as to why Abigail and Mercy might have run away.

PARRIS. I cannot think they would run off except they fear to keep in Salem anymore—since the news of Andover has broken here.

In a dramatic scene during the trial, John Proctor accuses Abigail of committing adultery with him. This is reason enough for Abigail to want to leave Salem: she wants to escape being ostracized for her illicit relationship with Proctor and to avoid the constant knowing, judgmental looks she's likely to receive from the community for the rest of the time she lives in Salem.

Abigail and Mercy were also complicit in accusing a number of people of witchcraft, and Abigail was guilty of sending innocent Salem residents to their deaths. Neither Abigail nor Mercy had yet been accused of any crimes related to their false testimony at the trials, but they might have decided that it was better for them to leave Salem before they became the subjects of trial proceedings themselves.

Rev. Parris mentions "the news of Andover," which refers to witch trials in Andover, Massachusetts, about fifteen miles northwest of Salem. Over forty Andover residents were accused of witchcraft, in much the same way as in Salem, and three of the accused were executed before the people of Andover rose up and brought an end to the witch trials. News of the uprising had apparently reached Salem, and this might be another reason that Abigail and Mercy decided to leave Salem, before they were caught up in an anti-witch trial movement in Salem.

In The Crucible, Arthur Miller portrays Abigail Williams as about seventeen years old and as a servant in the Proctor household, who has an affair with John Proctor. Abigail accuses John's wife, Elizabeth, of witchcraft so that she can marry John after Elizabeth is executed as a witch.

The real Abigail Williams was a girl of about eleven or twelve who lived with her uncle, Reverend Samuel Parris, and his family, possibly because Abigail's parents had died.

After Abigail's testimony on June 3, 1692, the day that John Willard and Rebecca Nurse were formally accused of witchcraft, Abigail Williams disappears from the historical record.

The Salem Witch Museum website offers this observation:

Even though Abigail played a major role as an accuser at the beginning of the trials, especially in March, April, and May, she gave her last testimony on June 3rd 1692. There is no historical documentation suggesting why Abigail virtually disappeared from the court hearings. In addition, there are no records indicating what happened to Abigail after the events of 1692. It is suggested that she never married and died a single woman, but without any evidence we will never be quite certain.

Arthur Miller used dramatic license in the character of Abigail Williams. In The Crucible, Miller changed Abigail’s age from eleven to seventeen. At the age of eleven, it was doubtful that John Proctor and Abigail had a relationship in real life. However, at the age of seventeen in the play, John and Abigail could have had an affair which caused her to be sent away from the Proctors. Elizabeth also was accused of witchery because of the jealousy and anger of Abigail.

Several things led to Abigail running away from Salem. The girls dance nude in the forest and study palmistry with Tituba, the West Indian slave. They are discovered by the local minister Reverend Parris.

Once the girls are found out, Parris sends for some help to figure out what was going on. It is then that the girls know that they have to move the spotlight from their action, or they could be accused of witchcraft.

Little good can be said of Abigail. She is Rev. Parris’s niece. She lies, seeks revenge, manipulates the court, and sought what is best for herself. The other girls follow her lead partly for fear of her wrath.

Led by Abigail, the girls begin having spells and fits and accusing various people throughout the community of worshipping the devil. From a homeless woman to the much admired Rebecca Nurse, the girls point their fingers at the people, and the court places convicted men and women in jail and sentence them to death.

John Proctor brought Mary Warren to court to accuse Proctor. In addition, the affair between the two of them is brought to the court with Williams denying it. After Proctor is sentenced to death and Rev. Hale denounces the court, the court and Salem begin to examine what is really going on with the girls.

On her behalf, Abigail watched as Indians killed her parents with tomahawk. Other aspects of her life hinder her from being a well behaved young lady. She is an orphan, a teenager, low social class, and female. Obviously, this is a girl in need of attention.

In Act IV, Rev. Parris tells Hale and Danforth that Abigail and another of the accusers Mercy Lewis have vanished from Salem. Parris thinks that they may have gotten on a ship. In addition, Abigail has stolen thirty-one pounds from Parris’ strongbox.

Parris gives the reason for Abigail’s escape:

I cannot think they would run off except they fear to keep in Salem any more---since the news of Andover has broken here…the rumor here speaks rebellion in Andover, and it…

The vicious nature of Abigail causes the death of twenty people. Others suffer in the jails for months until they are released and the court is disbanded. Abigail and Mercy are never seen or heard from again. It was a smart move on the part of Abigail because they probably would have landed in jail for their deceptions.

There are a few reasons for Abigail to leave at this point. It is clear that she and John will not have any kind of romantic relationship and Abigail has caused plenty of trouble for Elizabeth, her romantic rival. 

But the main reason she leaves is to avoid potential retribution. In the nearby town of Andover, the town rebelled against the court's charges of witchcraft. Parris tells Danforth that news of the Andover rebellion has been heard in Salem. Abby and Mercy most likely heard the news and left for fear that if Salem's people also unite against the court and get rid of the charges of witchcraft, she (Abby) could be blamed for inciting the whole thing from the start. 

Knowing that the Andover news has come to Salem, and that the proposed hangings of Rebecca Nurse and John Proctor might sway the town to object to the court, Parris fears a similar rebellion will occur in Salem. It is most likely that Abby and Mercy left Salem to avoid the consequences if a Salem rebellion were to occur. He pleads with Danforth to believe him: 

I tell you what is said here, sir. Andover have thrown out the court, they say, and will have no part of witchcraft. There be a faction here, feeding on that news, and I tell you true, sir, I fear there will be riot here.