In this chapter, Joab murders Amasa and crushes a second revolt.
The events of this chapter follow on immediately from the arguments that were breaking out in the previous chapter. In that chapter, the other tribes of Israel were complaining that the men of Judah had taken the king across the Jordan river and denied them any prominence or honor in re-establishing the Davidic kingdom. The men of Judah, in turn, responded to them "harshly", and that seems to have escalated the confrontation to a much more violent dispute.
In this chapter, the revolt begins with a Benjamite which is in many ways a continuation of the running conflict between Judah and Benjamin. To remind my readers, Saul was part of the tribe of Benjamin and he relentlessly sought to kill David. David, in turn, was part of the tribe of Judah and he sent gifts to the elders of Judah to ingratiate himself to them (1 Samuel 30:26). David fought against Saul's son, Ish-Bosheth, who eventually died.
When David was driven out of Jerusalem, a Benjamite named Shimei harassed him, which shows that the men of Benjamin are still pretty hostile to David. Now we see a Benjamite encouraging Israel to turn away from David, and what he says in particular is "We have no portion in David". Once again he is using the language of inheritance, which is because the men of Israel feel like Judah is trying to claim David as their own "inheritance", since he is their relative.
So that's the background for the conflict between David and Sheba.
When David returns, the first thing he does is take the ten widows who Absalom slept with and locks them up. David himself cannot sleep with them because the Law says that once a woman is divorced and marries another man, she cannot come back and marry the first man (Deut 24:1-4). This isn't the same thing, but to avoid the appearance of impropriety, David would seek to avoid sleeping with them again.
After this, David asks Amasa to call out the men, and what I feel here is David trying to empower Amasa and weaken Joab. Unfortunately for Amasa, he takes too long getting back so David starts getting desperate and turns to Abishai. Once again, David is trying to subvert Joab, but Abishai brings Joab with him, which makes sense because they are brothers and closely allied.
Joab once again takes matters into his own hands and slays Amasa in cold blood. Joab is generally loyal to David, but he is willing to kill anyone who gets in his way and challenges his authority. Joab and Abishai are more than happy to kill David's enemies too, and David is the one who prevents them. But in this case, Joab is fighting for his own position, and he is utterly ruthless when fighting for himself.
He is utterly ruthless when dealing with Sheba too. Sheba goes to hide in a city, and Joab goes to tear the city down in order to kill him. A "wise woman" prevents him, and from this story what I'd say is that Joab isn't trying to destroy towns of Israel, but he won't let that stop him if a town comes between him and his objective.
This chapter concludes by telling us the officials in David's administration, with the foremost being Joab himself. After killing Amasa, David again refrains from turning against Joab because he is afraid of Joab's influence. Even to the very end of David's life, he is never able to remove Joab from power or bring him to justice.
As unlikely as it may seem, this is the end of David's primary arc. 2 Samuel still has 4 more chapters after this, but the last 4 chapters are just a collection of unrelated stories taken from different periods of David's life. From this point on, it is entirely non-chronological and much more of an appendix.
So what can we say about David's life? I would describe David in a few ways.
First and foremost, he sincerely sought to follow the LORD and honor him as the true king of Israel. He brought the ark of the covenant back to Jerusalem with a triumphal procession and wrote many of the Psalms that are recorded later in the bible. He fought Goliath with a tremendous faith in God to save him and destroy his enemy.
Second, David was merciful to nearly everyone who resisted him or tried to harm him. David refused twice to kill Saul, refused to harm Ish-Bosheth, and Abner, and even Absalom, and Shimei, and he honored Mephibosheth, who was the grandson of his enemy Saul.
Third, David suffered on several occasions because he always saw the best in others and trusted others, which gave his enemies opportunities to betray him. Saul tried to murder him several times, and he returned after a couple of those murder attempts. All that did was give Saul more chances to "pin him to the wall" with a spear. David also forgave Absalom, brought him back into prominence, and then tried to spare his life even after Absalom betrayed him. David also refused to take action against Amnon, possibly out of a misguided sense of mercy.
Fourth, David was a survivor. For all of his mistakes, David perseveres through many challenges and hardship. He spent many cold nights tending sheep, and then hiding from Saul, and then later he was driven out of Jerusalem by his son Absalom.
Fifth, David made mistakes. The biggest mistake of his life was sleeping with Bathsheba and killing Uriah to conceal his sin. And he pays a heavy, heavy price for it. But he repents and tries to fix his life after that.
There's many more things that could be said, but for now I will leave my readers with these words. David was a human being, just like anyone else. Like him, we also face many challenges and hardships, like him we make mistakes and suffer for them, and like him, if we seek the LORD with our whole hearts, we will see great things in this life, and in the next, eternal life.