Matthew Henry on 2 Samuel 13:30–end
Here is, I. The fright that David was put into by a false report brought to Jerusalem that Absalom had slain all the king's sons, Sa2 13:30. It is common for fame to make bad worse; and the first news of such a thing as this represents it as more dreadful than afterwards it proves. Let us not therefore be afraid of evil tidings, while they want confirmation, but, when we hear the worst, hope the best, at least hope better. However, this false news gave as much affliction to David, for the present, as if it had been true; he tore his garments, and lay on the earth, while as yet it was only a flying story, Sa2 13:31. It was well that David had grace; he had need enough of it, for he had strong…
Read the whole note in the consoleJamieson-Fausset-Brown on 2 Samuel 13:30
tidings came to David, saying, Absalom hath slain all the king's sons--It was natural that in the consternation and tumult caused by so atrocious a deed, an exaggerated report should reach the court, which was at once plunged into the depths of grief and despair. But the information of Jonadab, who seems to have been aware of the plan, and the arrival of the other princes, made known the real extent of the catastrophe.
Adam Clarke on 2 Samuel 13:32
And Jonadab - said - Amnon only is dead - This was a very bad man, and here speaks coolly of a most bloody tragedy, which himself had contrived.
John Gill on 2 Samuel 13:33
Now therefore let not my lord the king take the thing to his heart,.... Neither suppose it, nor be troubled for it:
to think that all the king's sons are dead: which was not to be thought, nor could any good reason be given for such a supposition
for Amnon only is dead; he repeats it again with great assurance, which seems to confirm the suspicion of him before suggested; though some think he said this not from certain knowledge, but by conjecture.
Keil & Delitzsch