What is sin's purpose?

Sin aims always at the utmost; every time it rises up to tempt or entice, if it has its own way it will go out to the utmost sin in that kind. Every unclean thought or glance would be adultery if it could, every thought of unbelief would be atheism if allowed to develop. Every rise of lust, if it has its way reaches the height of villainy; it is like the grave that is never satisfied. The deceitfulness of sin is seen in that it is modest in its first proposals but when it prevails it hardens men’s' hearts, and brings them to ruin. John Owen (Of the Mortification of Sin in Believers)