2 Samuel 20:10
But Amasa took no heed to the sword that was in Joab’s hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struck him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri.
The verse belongs to the troubles of David that continued after the suppression of Absalom's revolt. A quarrel between the men of Judah and the men of Israel gave occasion for the revolt of Sheba the son of Bichri, which threatened to prove a very serious thing. Joab's killing of Amasa in this setting took place near Gibeon.
Joab killed Amasa by treachery under a show of friendliness, greeting him with a deceitful salute that hid a concealed sword and then striking him fatally under the fifth rib. The phrase under the fifth rib describes a treacherous attack that follows a show of friendliness. The assassination was especially guilty because Joab and Amasa were near relatives to David and to each other through Jesse, also called Nahash, the father of David, a relationship that explains many circumstances in their history.
The revolt and the killing stand among the calamities directly traceable to David's sin in the case of Uriah, as part of God's judgments upon him and proof that the sword would never depart from his house. They belong to the subsequent history of David after his great wickedness in the case of Uriah, by which he gave great occasion to the enemies of the Lord to blaspheme. They show how completely the terrible denunciation of the prophet Nathan was accomplished, a transgression that is not covered up or excused but placed on record with David's own humble confession. The chapter is also part of the account of troubles in David's family that grew out of his taking many wives. These troubles show that people cannot break God's laws without suffering, for even when God does not openly reprove and may seem to overlook wrongdoing, every transgression carries its own inherent curse.
The pleasure and prosperity of the wicked is just like Joab's salute to Amasa, pleasant on the outside but deceitful and deadly, and this prosperity slays them.