The protagonist in this chapter is really Abner, who was the commander of Saul’s army, and had really kept the family of Saul in power through Saul’s son Ishbaal after Saul’s death. But we begin the chapter by reading that Saul’s family was getting weaker and David’s kingdom was getting stronger. As a result Abner can read the writing on the wall and decides to make peace with David and swear allegiance to him so that the two kingdoms, Judah and Israel, can once again be reunited under one king, which would be king David.
David demands that if he were to agree to this then he wanted his old wife back, Michal, who was presently married to Paltiel. The king of Judah, Saul’s son, sends Michal back to David with Paltiel weeping behind her the whole way. This serial sinner David begins his exploits of taking women away from their husbands and claiming them as his own early on his kingdom. This is replayed when David takes and rapes Bathsheba.
So while Abner and David have a peace treaty signed not everyone is on board, especially that family who had lost loved ones in battle against Abner. This family, Joab and his brothers, coax Abner to a corner and kill him in cold blood. David is distraught and refuses to eat until the time of mourning is over which is supposed to show that he had nothing to do with the killing. The story will continue next chapter as Saul’s family continues to fall apart and David’s reign gets even stronger. So the glue that held Saul’s family together, Abner, is dead. Everything will now unravel.