Yup. The girls had to leave Salem, and it's not surprising they chose to leave together. They had to leave because the things they had done were not only crimes (committed perjury in the courtroom, made false accusations, lied) but also sins. Salem was a theocracy, which means their punishments would have been severe--and permanent. Arthur Miller's description of Abigail is apt--he says she has an endless capacity for dissembling (lying). He uses the word "sly" in his description of Mercy, making the two girls quite a pair.
If they were to stay in Salem after it had been discovered that they had lied all along, not only would they be ostracized, but quite likely they would be hung for it. Their only option was to leave the colony as fast as possible, never to come back.
I think I understand your question. At the end of the play, Abigail breaks into Rev. Parris's strongbox and steals all his money. Then she and Mercy run away from Salem before anyone realizes they are gone. The climate in regard to the witch trials had changed in Salem and the nearby villages. The people of Andover had thrown out the court, and the people in Salem were growing opposed to the court and no longer approved of the hangings. Their feelings seemed to change as more and more of the "respectable" people in Salem were accused. Abigail and Mercy fled Salem because they knew they had lost their influence and the tide had turned against them. They most likely feared that they would be held accountable for their actions that led to so many deaths and so much misery among their neighbors.
I'm sorry, but I do not really understand your question. I do not understand what you mean by "fear to keep Salem." I assume you have left out a word or two. Please let us know what you mean so we can give you a good answer.