Albert Barnes Commentary 2 Samuel 13:21

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

The Septuagint adds a helpful explanation: but he did not vex the spirit of Amnon his son, because he loved him, because he was his first-born. This lack of justice in David’s conduct, and favoritism to Amnon, probably rankled in Absalom’s heart and was the first seed of his subsequent rebellion.