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