Only Amnon is dead. This has been Absalom’s express intention ever since the day Amnon raped his sister Tamar

 2 Samuel 13:20-39

Two years later, when Absalom’s sheepshearers were at Baal Hazor near the border of Ephraim, he invited all the king’s sons to come there. 

Her brother Absalom said to her, “Has that Amnon, your brother, been with you? Be quiet for now, my sister; he is your brother. Don’t take this thing to heart.” And Tamar lived in her brother Absalom’s house, a desolate woman.

Then Absalom said, “If not, please let my brother Amnon come with us.”

The king asked him, “Why should he go with you?” But Absalom urged him, so he sent with him Amnon and the rest of the king’s sons.

 Absalom ordered his men, “Listen! When Amnon is in high spirits from drinking wine and I say to you, ‘Strike Amnon down,’ then kill him. Don’t be afraid. Haven’t I given you this order? Be strong and brave.” So Absalom’s men did to Amnon what Absalom had ordered. Then all the king’s sons got up, mounted their mules and fled.

But Jonadab son of Shimeah, David’s brother, said, “My lord should not think that they killed all the princes; only Amnon is dead. This has been Absalom’s express intention ever since the day Amnon raped his sister Tamar. My lord the king should not be concerned about the report that all the king’s sons are dead. Only Amnon is dead.”

Absalom fled and went to Talmai son of Ammihud, the king of Geshur. But King David mourned many days for his son.

After Absalom fled and went to Geshur, he stayed there three years. And King David longed to go to Absalom, for he was consoled concerning Amnon’s death.

Why?

David did not care about the effect of Tamar's rape of his sister Tamar, two years later, Absalom killed Amnon to revenge for his sister Tamar. The original fault comes from David doing nothing at home.

How?

Downfall come from our carelessness to sins around us. Be aware of our heart going on the word of the Lord.