What is "he'Hamon ha'Gadol?
Rashi: They were noisy and moving in war.
Why does it say "Lishlo'ach Es Eved ha'Melech Yo'av v'Es Avdecha"?
Rashi: The verse should be re-arranged. Yo'av sent the king's servant (the other herald) and your servant (myself).
How did this answer the question about Avshalom?
Radak: Because Yo'av rushed to send us, we did not delay to find out.
Did Achima'atz lie [when he said that he did not know the reason for the commotion]?
Malbim: Perhaps he accurately describes the start of his Shelichus. Right when Yo'av killed Avshalom, Achima'atz saw commotion and did not yet know what it is, and he suggested that he run and tell the king, and Yo'av said that it is not good news for the king.
