2 Samuel 13:21 - Joseph Benson’s Commentary on the Old and New Testaments

When David heard, he was very wroth With Amnon: whom yet he did not punish, at least so severely as he ought to have done; perhaps, because he was his eldest son, and the next heir to his crown, and therefore he was unwilling either to cut him off, or to expose him to contempt among the people he might hereafter be called to govern; or, because he could not punish him in any legal or equitable manner, without laying open the infamy of his house; or, which seems to have been the most weighty reason, because he was conscious of his own guilt, in an instance not very dissimilar, which certainly had set Amnon a bad example; and because he had otherwise been partly accessory to his guilt by a very unguarded compliance with his son's irrational request in sending Tamar to him. There can be no question but that David's guilt with Bath-sheba rendered him more backward to punish that of Amnon. “However, the guilt which human justice or human infirmity did not, or could not chastise as it deserved, the divine vengeance did.” Delaney.