9 And Joab saith to Amasa, 'Art thou 'in' peace, my brother?' and the right hand of Joab layeth hold on the beard of Amasa to give a kiss to him; 10 and Amasa hath not been watchful of the sword that 'is' in the hand of Joab, and he smiteth him with it unto the fifth 'rib', and sheddeth out his bowels to the earth, and he hath not repeated 'it' to him, and he dieth; and Joab and Abishai his brother have pursued after Sheba son of Bichri.
Matthew Henry's Commentary on 2 Samuel 20:9-10
Commentary on 2 Samuel 20:4-13
(Read 2 Samuel 20:4-13)
Joab barbarously murdered Amasa. The more plot there is in a sin, the worse it is. Joab contentedly sacrificed the interest both of the king and the kingdom to his personal revenge. But one would wonder with what face a murderer could pursue a traitor; and how, under such a load of guilt, he had courage to enter upon danger: his conscience was seared.