Amasa was not on guard against the dagger in Joab’s hand, and Joab stabbed him in the stomach and spilled out his intestines on the ground. And Joab did not need to strike him again, for Amasa was dead. Then Joab and his brother Abishai pursued Sheba son of Bichri.
We have here Amasa's fall just as he began to rise. He was nephew to David (Sa2 17:25), had been Absalom's general and commander-in-chief of his rebellious army, but, that being routed, he came over into David's interest, upon a promise that he should be general of his forces instead of Joab.
Commenting on 2 Samuel 20:4-13
But Amasa took no heed to the sword that was in Joab's hand,.... In his left hand, for with his right hand he took him by the beard; he might see the sword drop out of the scabbard, and Joab take it up, which he supposed he did in order to put it into its sheath again, having no suspicion of his wicked design, and...
AMASA IS SLAIN. (Sa2 20:10-13) smote him . . . in the fifth rib--the seat of the liver and bowels, where wounds are mortal. struck him not again--that is, despatched him at the first blow.