See J. B. Connolly, Master Mariner (1943).
Absalom, David's mutinous son, revolted and won over the tribes of Israel (). He appointed Amasa over the army, in effect replacing Joab, who had been general for his father David.
After the revolt was crushed, David held out an olive branch to Amasa (in a likely effort to appease and reunite Judah with the other 10 tribes of Israel) by offering to appoint Amasa as his new army commander (). While being fiercely loyal to David, Joab was also suspicious of any potential rivals for Joab's power or threats to David's kingdom, and had no qualms about taking the lives of any who might stand in his way (E.G., Abner: , and Absalom: ). So it was not difficult for Joab to also decide to murder Amasa ().