26 When Joab came out from David's presence, he sent messengers after Abner, and they brought him back from the cistern of Sirah; but David did not know about it. 27 When Abner returned to Hebron, Joab took him aside in the gateway to speak with him privately, and there he stabbed him in the stomach. So he died for shedding the blood of Asahel, Joab's brother. 28 Afterward, when David heard of it, he said, "I and my kingdom are forever guiltless before the Lord for the blood of Abner son of Ner. 29 May the guilt fall on the head of Joab, and on all his father's house; and may the house of Joab never be without one who has a discharge, or who is leprous, or who holds a spindle, or who falls by the sword, or who lacks food!" 30 So Joab and his brother Abishai murdered Abner because he had killed their brother Asahel in the battle at Gibeon.
Matthew Henry's Commentary on 2 Samuel 3:26-30
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.