2 Samuel 13:4
And he said unto him, Why art thou, being the king’s son, lean from day to day? wilt thou not tell me? And Amnon said unto him, I love Tamar, my brother Absalom’s sister. And he said unto him, Why art thou, being the king’s son, lean from day to day? wilt thou not tell me? And Amnon said unto him, I love Tamar, my brother Absalom’s sister.
Amnon was David's first-born son. He openly declared his love for Tamar, his brother Absalom's sister. That love led him to commit a shameful, great crime against her. The crime was adulterous, unnatural and incestuous, making him doubly guilty under the law that pronounced death upon the adulterer.
David, self-condemned by his own sin, failed to bring the offender to justice and permitted the crime to pass unpunished and unrebuked. Absalom, as the natural protector of his foully wronged sister, avenged her by having Amnon slain. He concealed his purpose of revenge for two full years, then had the drunken Amnon killed by his command at a feast of the king's sons.
The episode was bitter fruit of David's own wrongdoing, with his sons acting over the sins of which he himself had been guilty. In this twofold judgment David's own sentence upon himself was carried out. It kept David's sin continually before his mind and made him feel the full weight of his injustice to Uriah and Bathsheba.