Joab, man of blood

At 2 Samuel 19 Joab had already murdered Abner, an honorable and decent man during peacetime, 2 Samuel 3:26-27, so we know that he wasn't a very nice man. David then had to navigate the real world politics of having an evil man of blood who was interested in his own personal position of power working for him, and someone he couldn't depose. Knowing his bloodthirsty character David even used him to kill the twice honorable Uriah the Hittite.
Joab then beings to plot against David through his son Absalom, hedging his bets as it were since Absalom was his kind of king (2 Samuel 14) but Absalom decided against him (2 Samuel 17:25), forcing Joab to support David in order to keep his position during the rebellion which he helped stir up.
It is not surprising then to read the following:
2 Samuel 19:5,7- "Then Joab came into the house to the king and said, "You have today covered with shame the faces of all your servants, who have this day saved your life and the lives of your sons and your daughters and the lives of your wives and your concubines...Now therefore arise, go out and speak kindly to your servants, for I swear by the LORD, if you do not go, not a man will stay with you this night, and this will be worse for you than all the evil that has come upon you from your youth until now."
This isn't a prophecy of what may happen, this is a statement of borderline insurrection. Translation: "If you don't celebrate the death of that rebellious son if yours I'm going to take the army from you, leaving you without a kingdom. Absalom rebelled against you and had some success, I guarantee that if you don't obey me I will succeed in overthrowing your life."

In light of this David has had enough, Joab has to go. He appoints Amasa, Absaloms general to lead his forces from now on (2 Samuel 20:4) to show those who followed his son he didn't hold a grudge against them, and to remove the threat to his person.
Predictably Joab kills Amasa in cold blood, (2 Samuel 20:9-10) usurps his title back (2 Samuel 20:11).
In fact, his conscious was so seared, that he thought nothing of Amasa laying on the road in a pool of his own blood dying, but his own soldiers were not happy about this (2 Samuel 20:12).

Even with all this, Joab may have had a decent remainder of his life, but he chose to defy David and support the rebellion of Adonijah against Solomon, for which Solomon finally put him to death for treason. (1 Kings 2:28).
And the record of the man of blood came to an end, as do all men who love evil and defy goodness.