Since her parents were killed, Abigail has spent her life as a poor relation in the house of the Reverend Parris. This cannot have been an easy or agreeable position, but it has given her a first-class training in the arts of dissimulation. Abigail has had to lie to her uncle regularly and often, to the point that she now lies by default and avoids the truth as a matter of preference.
When Hale is questioning Tituba, Abigail sees an opportunity to deflect attention from herself and her dancing in the woods. She pretends to repent of her own sin in dancing for the devil and then moves on, with all possible speed, to accusing others.
It may be that Abigail has already noticed Betty stirring when she begins her string of accusations. In any case, Betty quickly "picks up the chant," as Miller's notes direct, lending support to Abigail's accusations. Deflecting attention from her own transgressions is clearly Abigail's principal motive—and it may initially be her only one—but she is devious enough to consider other reasons for making these accusations, even in act I.
Abigail has tasted power in the effect her beauty has on John Proctor, and it is clear that she enjoys being powerful. He accusations make her, for a brief period, the most powerful person in Salem. She manipulates even the judges, Danforth and Hathorne. She has also been humiliated by the people of Salem for her tendency to laugh in church and for her hasty departure from the Proctor household. It cannot have been long before Abigail realized that these accusations were a perfect way of taking revenge on those who slighted her. Both these motives could have been in her mind from the very beginning, along with her determination to save herself.