26 And when Joab had come out from David, he sent men after Abner, and they overtook him at the water-spring of Sirah, and made him come back with them: but David had no knowledge of it. 27 And when Abner was back in Hebron, Joab took him on one side by the doorway of the town to have a word with him quietly, and there he gave him a wound in the stomach, causing his death in payment for the death of his brother Asahel. 28 And when David had word of it he said, May I and my kingdom be clear for ever in the eyes of the Lord from the blood of Abner, the son of Ner: 29 May it come on the head of Joab and all his father's family: among the men of Joab's family may there ever be some who are diseased or lepers, or who do the work of women, or are put to the sword, or are wasted from need of food! 30 So Joab and Abishai his brother put Abner to death, because he had put to death their brother Asahel in the fight 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.