25 You know Abner the son of Ner, that he came to deceive you, and to know your going out and your coming in, and to know all that you do.” 26 When Joab had come out from David, he sent messengers after Abner, and they brought him back from the well of Sirah; but David didn’t know it. 27 When Abner was returned to Hebron, Joab took him aside into the midst of the gate to speak with him quietly, and struck him there in the body, so that he died, for the blood of Asahel his brother. 28 Afterward, when David heard it, he said, “I and my kingdom are guiltless before Yahweh forever of the blood of Abner the son of Ner. 29 Let it fall on the head of Joab, and on all his father’s house. Let there not fail from the house of Joab one who has an issue, or who is a leper, or who leans on a staff, or who falls by the sword, or who lacks bread.” 30 So Joab and Abishai his brother killed Abner, because he had killed their brother Asahel at Gibeon in the battle.
Matthew Henry's Commentary on 2 Samuel 3:25-30
Commentary on 2 Samuel 3:22-39
(Read 2 Samuel 3:22-39)
Judgments are prepared for such scorners as Abner; but Joab, in what he did, acted wickedly. David laid Abner's murder deeply to heart, and in many ways expressed his detestation of it. The guilt of blood brings a curse upon families: if men do not avenge it, God will. It is a sad thing to die like a fool, as they do that any way shorten their own days, and those who make no provision for another world. Who would be fond of power, when a man may have the name of it, and must be accountable for it, yet is hampered in the use of it? David ought to have done his duty, and then trusted God with the issue. Carnal policy spared Joab. The Son of David may long delay, but never fails to punish impenitent sinners. He who now reigns upon the throne of David, has a kingdom of a nobler kind. Whatever He doeth, is noticed by all his willing people, and is pleasing to them.