Albert Barnes Commentary 2 Samuel 13:32

"And Jonadab, the son of Shimeah, David`s brother, answered and said, Let not my lord suppose that they have killed all the young men the king`s sons; for Amnon only is dead; for by the appointment of Absalom this hath been determined from the day that he forced his sister Tamar." — 2 Samuel 13:32 (ASV)

The history supplies another instance of Jonadab’s subtlety and sagacity (compare 2 Samuel 13:3). He immediately gave the true explanation of the catastrophe at Baal-hazor, despite the false rumor.

By the appointment of Absalom... This means that Absalom’s resolution to slay Amnon had been formed at that time, and he was only waiting for an opportunity to carry it out.