Abigail Williams is as deeply involved as the other girls in the weird cavortings that took place in the forest that night. She seeks to absolve herself of all responsibility for her actions by placing the blame on others. Abby figures there's no point in denying what happened; instead, she focuses on distancing herself from the strange goings-on in which she was an active participant. Crazy dancing is one thing, she can admit to that. But conjuring up evil spirits is another matter entirely, and Abby doesn't want to be associated with this diabolical activity.
Abby is worried that Ruth and Tituba might implicate her in what happened that night in the forest, so she acts first. By pointing the finger of suspicion at the other girls, Abby puts the onus on them to prove they weren't involved in anything that could be interpreted as witchcraft. This allows her to maintain a facade of innocence, acting as if butter wouldn't melt in her mouth, while at the same time distancing herself from the part she played in messing around with the forces of darkness.