Elizabeth realizes that Abigail not only wants her gone and out of John's life, she actually wants her dead.
Mary Warren refuses to tell Elizabeth who actually accuses her as she is "bound by law." But Elizabeth hardly needs to know the name. She is sure it is Abigail, and here is why:
Elizabeth: She wants me dead. All week I knew this would come...She will cry me out until they take me!...She wants me dead, John, you know it...She thinks she can take my place, John.
What Elizabeth means is that Abigail is so consumed with lust for John that she will do absolutely anything to be rid of Elizabeth, even have her killed so that she can be with him. Because Abigail and John had slept together, Abigail feels that he has made her a "promise" to be with her and leave his wife. She is tired of waiting. Elizabeth knows this, but John refuses to see.