Throughout the war between Saul's house and David's house, Abner was gaining power in Saul's house.
References for 2 Samuel 3:6
Now Saul had a secondary wife named Rizpah, Aiah's daughter. Ishbosheth said to Abner, "Why have you had sex with my father's secondary wife?"
References for 2 Samuel 3:7
Abner got very angry over what Ishbosheth had said. "Am I some sort of dog's head?" Abner asked. "I've been nothing but loyal to the house of your father Saul and to his brothers and his friends. I haven't handed you over to David, but today you accuse me of doing something wrong with this woman.
References for 2 Samuel 3:8
May God deal harshly with me, Abner, and worse still if I don't do for David exactly what the LORD swore to him—
removing the kingdom from Saul's house and securing David's throne over Israel and over Judah, from Dan all the way to Beer-sheba!"
Ishbosheth couldn't say a single word in reply to Abner because he was afraid of him.
Abner sent messengers to represent him to David and to say, "Who will own the land? Make a covenant with me, then I'll help bring all Israel over to your side."
References for 2 Samuel 3:12
"Good!" David replied. "I will make a covenant with you, but on one condition: don't show yourself in my presence unless you bring Saul's daughter Michal when you come to see me."
Then David sent messengers to Saul's son Ishbosheth. "Give me my wife Michal," he demanded. "I became engaged to her at the cost of one hundred Philistine foreskins."
Ishbosheth then sent for Michal and took her from her husband Paltiel, Laish's son.
Her husband went with her all the way to Bahurim, crying as he followed her. "Go home!" Abner told him. So he went home.
Abner then sent word to Israel's elders. "You've wanted David to be your king for some time now," he said.
"It's time to act because the LORD has said about David: I will rescue my people Israel from the power of the Philistines and all their enemies through my servant David."
Abner also spoke directly to the Benjaminites. He then went to inform David in person at Hebron regarding everything that all Israel and the house of Benjamin were willing to do.
When Abner, along with twenty others, reached David at Hebron, David threw a celebration for Abner and his men.
Then Abner said to David, "Please let me get going so I can assemble all Israel for my master the king. Then they can make a covenant with you, and you will rule over everything your heart desires." At that, David sent Abner off in peace.
References for 2 Samuel 3:21
Right then, David's soldiers and Joab returned from a raid, bringing a great deal of loot with them. Abner was no longer with David in Hebron because David had sent him off in peace.
When Joab and all the troops with him returned, Joab was told that Abner, Ner's son, had come to the king and that David had sent him off in peace.
Joab went to the king and asked, "What have you done? Abner came to you here! Why did you send him off? Now he's gotten away!
Don't you know the evil ways of Abner, Ner's son? He came to trick you, to find out where you come and go, and to learn everything you do!"
References for 2 Samuel 3:25
Joab left David and sent messengers after Abner. They brought him back from the well at Sirah, but David didn't know anything of this.
When Abner returned to Hebron, Joab took him aside next to the gate to speak with him in private. But instead Joab stabbed Abner in the stomach, and he died for shedding the blood of Asahel, Joab's brother.
References for 2 Samuel 3:27
When David heard about this later, he said, "I and my kingdom are forever innocent before the LORD concerning the shedding of the blood of Abner, Ner's son.
May it fall upon the head of Joab and his entire family tree! May Joab's family never be without someone with a discharge or a skin disease, someone who uses a crutch, someone who dies by the sword, or someone who is hungry!"
References for 2 Samuel 3:29
So that is how Joab and his brother Abishai murdered Abner, because he killed their brother Asahel in the battle at Gibeon.
Then David ordered Joab and all the troops who were with him, "Tear your clothes and put on funeral clothes! Mourn for Abner!" King David himself walked behind the body.
They buried Abner in Hebron. The king wept loudly at Abner's grave. All the troops cried too.
Then the king sang this funeral song for Abner: "Should Abner have died like a fool dies?
References for 2 Samuel 3:33
Your hands weren't bound, your feet weren't chained, but you have fallen like someone falls before the wicked." Then the troops cried over Abner again.
Then all the soldiers came to urge David to eat something while it was still day, but David swore, "May God deal harshly with me and worse still if I eat bread or anything else before the sun goes down."
All the troops took notice of this and were pleased by it. Indeed, everything that the king did pleased them.
So on that day all the troops and all Israel knew that it wasn't the king's idea to kill Abner, Ner's son.
The king told his soldiers, "Don't you know that a prince and a great man in Israel has fallen today?
And today, though I am the anointed king, I am weak. These men, Zeruiah's sons, are too strong for me. May the LORD repay the one who does evil according to the evil they did!"
References for 2 Samuel 3:39