What Young is trying to explain here is that it is not necessary to repent for your sins. All people are born with a natural set of morals, giving them the ability to tell right from wrong. Someone who has sinned knows that their actions were wrong, but they chose to do them anyway. Those sins will follow them for the rest of their life, so their is no need for them to be given any extra punishment.
Guilt is its own punishment. Young's description of sin "devouring you from the inside" is quite accurate, but seems to apply more to the guilt that goes along with the sin. Guilt is something that a person can not let go of, and it is a very difficult thing to fix. Holding on to the knowledge that you have sinned or committed a crime will take over your life, and you will never be free of it. The reader does not know how the murderer dealt with his knowledge of his sins, but it is known that Mack becomes withdrawn as he struggles with the idea that he has sinned. He thinks that Missy's disappearance and death are a punishment for sins he committed against his father, not respecting him and not keeping a good relationship with him. Now, Mack knows that Missy's death was not a punishment and not his fault. His punishment was the fact that he spent years feeling guilty about the way he had treated his father and worried about the repercussions.
Guilt is a very interesting concept, and it often shows up in literature. The feelings of guilt seem to be stronger than any other feeling, which is why many writers focus on it. As the reader follows Mack through his journey, they can understand his pain as he feels guilt for his past actions; they can empathize with him because everyone has felt guilt at one point or another. Guilt is something that one cannot forget about. It cannot be wiped away and it is always present. This appears also in The Scarlet Letter by Nathaniel Hawthorne as Dimmesdale is devoured by his guilt. He becomes weaker and weaker because he cannot live with the sin he has committed. This is what sin and the guilt of committing that sin does to a person; it ruins them. Therefore, there is no need for further punishment; their life is their punishment.