2 Then David deployed his troops, a third under Joab, a third under Abishai son of Zeruiah, Joab's brother, and a third under Ittai the Gittite. The king then announced, "I'm marching with you."

Matthew Henry's Commentary on 2 Samuel 18:2

Commentary on 2 Samuel 18:1-8

(Read 2 Samuel 18:1-8)

How does David render good for evil! Absalom would have only David smitten; David would have only Absalom spared. This seems to be a resemblance of man's wickedness towards God, and God's mercy to man, of which it is hard to say which is most amazing. Now the Israelites see what it is to take counsel against the Lord and his anointed.