2 Samuel 13:21

When king David heard of all these things, he was very wroth— We may easily conceive what resentment and uneasiness David felt for this crime: but how he punished it we know not. The truth is, he could not punish it without exposing the infamy of his house, and cutting off his eldest son: and how hard was it for a father to do this. The LXX and the Vulgate, whom Houbigant follows, add these words to the present verse: David, when he knew all these things, was very much afflicted: but he would not grieve the spirit of his son Amnon, for he loved him exceedingly, as being his first-born. There can be no doubt but that David's consciousness of his own guilt with Bath-sheba, rendered him more backward to punish that of Amnon. However, the guilt which human justice or human infirmity did not chastise as it deserved, the divine vengeance did.