The best way to figure out what Marry Warren felt about all of the accusations is to look at her own words and accounts of the events. The first time that we see her, she expresses horror that she and her friends might be accused of witchcraft. She worries, "witchery's a hangin' error...we must tell the truth!" She knows that if they themselves are accused of witchcraft and found guilty, they could be hanged. So, she has a massive fear of being punished for the possible dancing and spell-casting that was occuring in the woods. She goes on to point out that she "never done none of it" but only watched as the others did. Already she is trying to get out of trouble.
Later however, as all of the girls get off scot-free through accusing others of being witches, Mary joins the bandwagon. We see her true feelings on this in act two as she comes home and tells them that one woman, Goody Osburn, will be hanged because of her testimony against her. She describes seeing Goody Osburn, and getting all cold and tingly and not being able to breathe, and then all of a sudden remembering everything that Osburn had done to her. So at this point, Mary believes in witches, and believes that she has helped point one out to the courts. She felt bodily discomfort, and attributed it to witchcraft. She is still upset that a woman will be hanged for it though.
Later, in act three, we see her turn back on this belief. She denies being bewitched and tells the court that it were "all pretense." She goes on to elaborate, saying that she heard all of the other girls screaming and showing symptoms so it was super easy to believe she had them too. But, she didn't; she was just caught up on the moment. She knows that it is a fraud, and is trying to come clean. But as Abby turns on her, she chickens out, and against her better judgment, goes back to the dark side.
So, Mary Warrn starts out believing in witches, and being afraid to be called one; later, she feels like it is possibly real. In the end however, she knows that it is not, but goes along with it anyway as she caters to her own fears of being accused. I hope that those thoughts helped; good luck!