Why is Mary Warren afraid of telling the truth about Abigail, for herself and for Mr. Proctor?

Marry is petrified of Abigail who said that anybody who crosses her would meet with a "pointy reckoning". Abigail is also after John's wife Elizabeth who she blames for keeping John away from her and "blackening" her name in town. She also says that Abigail will charge him with lechery.