Charles Ellicott Commentary 2 Samuel 13:21

"But when king David heard of all these things, he was very wroth." — 2 Samuel 13:21 (ASV)

He was very angry. —The Septuagint adds, “but he vexed not the spirit of Amnon his son, because he loved him, because he was his firstborn,”—which is doubtless in part the reason for David’s guilty leniency. The remembrance of his own sin also tended to withhold his hand from the administration of justice. David’s criminal weakness towards his children was the source of much trouble from this time to the end of his life.