25 Thou knowest Abner the son of Ner, that he came to deceive thee, and to know thy going out and thy coming in, and to know all that thou doest. 26 And when Joab was come out from David, he sent messengers after Abner, and they brought him back from the well of Sirah: but David knew it not. 27 And when Abner was returned to Hebron, Joab took him aside into the midst of the gate to speak with him quietly, and smote him there in the body, so that he died, for the blood of Asahel his brother. 28 And afterward, when David heard it, he said, I and my kingdom are guiltless before Jehovah for ever of the blood of Abner the son of Ner: 29 let it fall upon the head of Joab, and upon all his father's house; and let there not fail from the house of Joab one that hath an issue, or that is a leper, or that leaneth on a staff, or that falleth by the sword, or that lacketh bread. 30 So Joab and Abishai his brother slew 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.