The Question and Answer section for The Crucible is a great resource to ask questions, find answers, and discuss the novel.
We learn that the play is based off of McCarthisim in the United States during the 1950's.
The best answer to your question would be jealousy, as they were one of the most respected couples in Salem. Another answer would be Mrs. Putnam's belief that there was an underlying reason for the deaths of her children.... she blamed Rebecca...
Mary is terrified of Abigail and her threats. She knows that if she testifies the other girls will turn the tables and accuse her of being a witch as well.
"They'll turn on me...I cannot do it!"