John Proctor is a noble man because he puts his commitment to the truth above all else, tho it will cost him a chance to live on with his wife and to know his child who is as yet unborn. I see him as an imperfect person who has 'sinned' by taking a lover outside of marriage, but also as a person with enough of a conscience to be unable to go on living a lie. It clearly eats at him , his betrayal of his wife, and his internal morality knows that he could not go on after telling another whopper.
John Proctor is literally willing to be executed rather than save himself by uttering untruth. Of course, his belief in an afterlife motivates him, as well, and he is thinking about AFTER he dies, and having to answer to a God that demands that he do his best to live a moral life on earth.