John Proctor is a character who was riddled with guilt over his affair with Abagail Williams, and his relationship with his wife became strained as a result. Proctor is defensive and bristles at even the suggestion (or perceived suggestion) of his past transgression. Elizabeth says to Proctor "I do not judge you. The magistrate that sits in your heart judges you." His sin of adultery has caused major conflicts within Proctor's heart, and he seems to be a man who walks around with a dark cloud over his head. He wonders if he is a good man, and struggles with the fact that he feels a sinner in the eyes of his wife, but is perceived to be a just and moral man in the eyes of the community. It is this secret and its accompanying guilt that he and is wife harbor that, like a cancer, causes his soul to waste away. It is not until the very end, when he tears up his confession, that he finally feels good and just because he knows he is doing the right thing in going to the noose with his good name intact. In a final act of pure love, Elizabeth declines to save the life of her husband because he finally has achieved the peace and self-assurance that he had been searching for, and "God forbid she should take it from him."
Elizabeth says this right at the end of the play. John has struggled with the question of whether to lie in order to save his life. He has finally decided that he will not lie.
At that point, he finally feels good about himself. He had always questioned whether he was really a good person. Now he finally has done something that has convinced him that he is.
So Elizabeth doesn't want to tell him to lie to save himself. She knows this would take away the feeling of goodness. She thinks it would be better for him to die feeling good about himself than to live and still think of himself as no good.
She doesnt want to take the goodness that he has away from him. If he doesnt want him to lie then she will no make him lie for her nor anyone else.
Elizabeth says that meaning John has become righteous by tearing up his confession and she shouldn't chase after him and beg him to make yet another confession.