This is essentially a rehash of number 8. We’re looking at a story where David was failing as a leader and a father. David knew there was troubles in his kingdom; from within his family itself. But he wasn’t dealing with them. This particular incident is another instance of literal history, not a condoning of the action by God.
David’s son, Amnon, was ‘in love’ with Thamar. He was really lusting after her. It was consuming him. Ammon with the help of another devised a plan in which Ammon would take advantage, or rape, Thamar. The executed the plan and they succeeded.
Nowhere does it say that God approved of the action. The only way that one might suggest that the Lord had approved of the action is that the Lord didn’t kill Ammon himself. And the Lord didn’t punish, at least directly, David for not taking care of the situation. David knew of the incident but didn’t do anything about.
Because of God providing us with the story, we get insight into what happens when you let sin run rampant in your household. Sin ran in David’s household and there was a big price to pay later down the road as it wasn’t addressed when it should have been. Even though God tells us that David was a man after His own heart, David wasn’t given a free pass, just as we are not. David surely did sin. Big time. Because God showed us the history, we can see the lesson; don’t put off or wait to correct sin.