Throughout act three, Abigail's reputation as a holy witch accuser is nearly overthrown as both Mary Warren and John Proctor cast doubt upon her reputation. Mary Warren asserts that she, Abby and the other girls were all pretending to be afflicted by witches, when they weren't. It is at this point that Judge Danforth turns to her and demands to know if that is true. Her response is highly defensive, elusive, and then, accusatory. She retorts,
"Let you beware, Mr. Danforth. Think you to be so mighty that the power of Hell may not turn your wits? Beware of it!"
So, in her defensiveness and desperacy to turn the blame from herself, she attacks the high judge, questioning if the judge is safe from the devil. She suggests that even he could become a servant of satan, and to beware of that. This is highly dangerous and risky to do; Danforth is a proud man, and holds her life in his hands. To accuse HIM of witchcraft is highly offensive and dangerous. She distracts him from the charge--probably realizing how risky it was--by then pretending, again, to bewitched, this time by Mary Warren. It is a diversionary tactic, and gratefully, Danforth is distracted from her inflammatory charge against him.
I hope that helped; good luck!