All throughout the play, John Proctor has thought of himself as a "fraud," a word he uses to describe himself in the fourth act when he is considering whether or not to confess to witchcraft and save his life. He cheated on his wife with their household help, a seventeen year-old girl named Abigail Williams, and then he failed to tell the court what he knew about her deception until after it was too late and hysteria had taken hold of Salem. In one sense, it is absolutely possible to consider him at least partially responsible for the tragedies that take place in the text.
In the end, John asks his wife, Elizabeth, to forgive him for what he's done. He wants her to absolve him, but she sagely says that her forgiveness would not matter "if [he'll] not forgive [him]self." She confesses that she, herself, feels guilty for her behavior within their marriage, and they have such a beautiful moment of truthfulness and love that John wants to stay alive to be with her, and he says that he will confess to save his life. However, he must first lie, and do so in front of Rebecca Nurse, whose conscience would not permit her to lie. Then he is asked to name other witches, but he refuses. Next he is forced to sign the document on which his untrue confession has been recorded. He learns it will be made public, and he wonders how he can teach his sons to be men if he "sold [his] friends." Suddenly, in tears, he crumples the confession and tears it up, telling his persecutors,
You have made your magic now, for now I do think I see some shred of goodness in John Proctor. Not enough to weave a banner with, but white enough to keep it from such dogs.
He has finally learned to see himself as a good man, someone who can be redeemed by keeping his integrity, but he can only do so by refusing to lie now. Therefore, he tears up his confession.