25 Surely you realize that Abner the son of Ner came to deceive you, to know your going out and your coming in, and to know all that you are doing." 26 And when Joab had gone from David's presence, he sent messengers after Abner, who brought him back from the well of Sirah. But David did not know it. 27 Now when Abner had returned to Hebron, Joab took him aside in the gate to speak with him privately, and there stabbed him in the stomach, so that he died for the blood of Asahel his brother. 28 Afterward, when David heard it, he said, "My kingdom and I are guiltless before the Lord forever of the blood of Abner the son of Ner. 29 Let it rest on the head of Joab and on all his father's house; and let there never fail to be in the house of Joab one who has a discharge or is a leper, who leans on a staff or 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.