What does sin cause?

It causes you to not realize when situations are out of control. It causes you to not recognize the problems you are surrounded. It causes you not to take action against sins that should be punished.

King David's son Amnon had relations with his half sister Tamar (2 Samuel 13). It mentions here that David was angry when he heard what happened. That is it though. David does not do anything else. In fact it seems that later when Absalom plans to kill Amnon that David has no idea that Absalom might want to do this.

David's unwillingness (or inability) to appropriately punish Amnon for his sin ends up leading to Absalom's killing of Amnon. All of this goes back to David's original sin with Bathsheba. This sin lead to a series of problems that come into David's kingdom.

Another interesting relationship David had with a sinful man was David's relationship with Joab.