1) Abigail and Mercy flee because Abigail can no longer have what she wants.
2) Both Abigail and Mercy are afraid of retribution that will come to Salem in the same way that it came to the town of Andover.
3) They are both afraid for their lives so they run away.
You can also infer that Abigail and Mercy left Salem because they feared the villagers would turn on them.
