2 Samuel 13:2
And Amnon was so vexed, that he fell sick for his sister Tamar; for she was a virgin; and Amnon thought it hard for him to do any thing to her. And Amnon was so vexed, that he fell sick for his sister Tamar; for she was a virgin; and Amnon thought it hard for him to do any thing to her.
Amnon, the one vexed over Tamar, was the first-born son. His being vexed and sick was lust for his virgin sister Tamar, the beginning that led to a shameful, incestuous, unnatural great crime. The law pronounced death upon the adulterer, and the unnatural character of his crime made Amnon doubly guilty.
His crime was bitter fruit of David's wrongdoing, with David's sons acting out the sins he had committed. This kept David's sin continually before him and made him feel the full weight of his injustice to Uriah and Bath-sheba. Though guilty, Amnon was left unpunished and unrebuked because David, self-condemned for his own sin, failed to bring him to justice.
Absalom, as the natural protector of his foully wronged sister, concealed his purpose of revenge for two full years. Then he had the drunken Amnon slain by his command at a feast of the king's sons. In this twofold judgment was meted out to David according to his own sentence.