The quote is a form of direct characterization. Miller is telling the reader something about John, and Miller isn't hinting. He is telling the audience that John Proctor is a sinner. A sinner against the moral laws of the Puritans and against his own vision of what he should be like. The quote that you provided is one piece of the puzzle, but the quote is further helped a few lines later with the following:
"Proctor, respected and even feared in Salem, has come to regard himself as a kind of fraud."
John is a sinner against the moral fashion of the time, because he had an affair with Abigail Williams. John is a sinner against his own vision of decent conduct, because he knows what he did, but nobody else knows. He is held in high regard by everyone in Salem, but he believes it is a falsity. In a way, John feels as if he is lying to everybody about his goodness. He feels like a hypocrite, which is why Miller writes that John was a sinner "against his own vision of decent conduct."
The hardest part about John is seeing that he is truly a good man. He sinned against his wife yes, and he feels horribly guilty for it too. But he lets his guilt consume him, and he is never able to focus on and build on the things that do make him a good man to the rest of Salem.