Elizabeth realizes that Abigail is hell-bent on destroying her and using a false accusation of witchcraft as the instrument of her destruction. Abigail is so vindictive, so completely consumed by hate, that she's prepared to do whatever it takes to ensure that Elizabeth ends up dangling on the end of a rope. With Abigail, this is personal, as Elizabeth now realizes to her total horror. Abigail still harbors feelings for John Proctor, as she makes perfectly clear to him.
But although the witch craze has given Abigail enormous power and prestige, the one thing it hasn't given her—and can't give her—is John himself. John knows he made a huge mistake in sleeping with Abigail, and he's so wracked by guilt that he's not about to do it again. His rejection of Abigail infuriates her, and so she sets out to do as much damage as she possibly can to her former lover, starting with the deliberate, cold-blooded destruction of his wife. Elizabeth knows what Abigail is up to. The realization of this terrifies her, and with good reason, too.
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.