25 Thou hast known Abner son of Ner, that to deceive thee he came, and to know thy going out and thy coming in, and to know all that thou art doing.' 26 And Joab goeth out from David, and sendeth messengers after Abner, and they bring him back from the well of Sirah, and David knew not. 27 And Abner turneth back to Hebron, and Joab turneth him aside unto the midst of the gate to speak with him quietly, and smiteth him there in the fifth 'rib'—and he dieth—for the blood of Asahel his brother. 28 And David heareth afterwards and saith, 'Acquitted 'am' I, and my kingdom, by Jehovah, unto the age, from the blood of Abner son of Ner; 29 it doth stay on the head of Joab, and on all the house of his father, and there is not cut off from the house of Joab one having an issue, and leprous, and laying hold on a staff, and falling by a sword, and lacking bread.' 30 And Joab and Abishai his brother slew Abner because that he put to death Asahel their brother, in Gibeon, in 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.