He smote him— This action was attended with the highest perfidy and insolence. Many reasons concurred to prevent David's calling him to an account now; particularly his power, authority, and interest with the army: but it is plain that he never forgot this outrage of Joab's. 'That he highly resented it, we find in his last charge to Solomon, where he recommends and gives it in charge to his son to do justice on that bloody assassin. He was not now in a capacity to do it. Joab was too powerful a subject to be brought to account. After Absalom's death, he had the insolence to tell the king with an oath, that he would make every one of his subjects desert him; and after this assassination of Amasa, he resumed, contrary to his master's will, the command of all the forces, who had an affection for him as their own general, because he had restored the quiet of the land by quelling the insurrection of Sheba. Joab's being continued captain-general of all the forces, was not from David's inclination, but contrary to it, and by force.
2 Samuel 20:10-13
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 struckd him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri.
11 And one of Joab's men stood by him, and said, He that favoureth Joab, and he that is for David, let him go after Joab.
12 And Amasa wallowed in blood in the midst of the highway. And when the man saw that all the people stood still, he removed Amasa out of the highway into the field, and cast a cloth upon him, when he saw that every one that came by him stood still.
13 When he was removed out of the highway, all the people went on after Joab, to pursue after Sheba the son of Bichri.