As unbearably difficult as it is for Proctor to leave his life, marriage, and children, he seems to believe that there is no way forward for him in Salem after the witchcraft trials. He also believes that Elizabeth will be executed after she gives birth and that the destruction of their family is an inevitability.
John Proctor understands that to give a false confession to save his life would do at least four things that he would not be able to live with. First of all, some people would believe that he actually did practice witchcraft. Secondly, those who did not believe he practiced witchcraft and gave the confession simply to save his life would look down on him for renouncing his faith in God. In a Puritan society, a person's reputation is one's most valuable asset. Next, in his Puritan belief, his confession would be a lie and thus an affront to God. And finally, to give a false confession would lend the trials legitimacy, and Proctor would not want to do that. He recognizes the corruption that Abigail and the girls have unleashed, and he wants no part of the mechanism that people in Salem are using to bring down their neighbors.