Just then David's men and Joab returned from a raid and brought with them a great deal of plunder. But Abner was no longer with David in Hebron, because David had sent him away, and he had gone in peace.
References for 2 Samuel 3:22
When Joab and all the soldiers with him arrived, he was told that Abner son of Ner had come to the king and that the king had sent him away and that he had gone in peace.
So Joab went to the king and said, "What have you done? Look, Abner came to you. Why did you let him go? Now he is gone!
You know Abner son of Ner; he came to deceive you and observe your movements and find out everything you are doing."
Joab then left David and sent messengers after Abner, and they brought him back from the well of Sirah. But David did not know it.
Now when Abner1 returned to Hebron, Joab took him aside into the gateway, as though to speak with him privately. And there, to avenge the blood of his brother Asahel, Joab stabbed him2 in the stomach, and he died.3
References for 2 Samuel 3:27
Later, when David heard about this, he said, "I and my kingdom are forever innocent4 before the LORD concerning the blood of Abner son of Ner.
References for 2 Samuel 3:28
May his blood5 fall upon the head of Joab and upon all his father's house!6 May Joab's house never be without someone who has a running sore7 or leprosy or who leans on a crutch or who falls by the sword or who lacks food."
References for 2 Samuel 3:29
(Joab and his brother Abishai murdered Abner because he had killed their brother Asahel in the battle at Gibeon.)