25 You know Abner son of Ner better than that. This was no friendly visit. He was here to spy on you, figure out your comings and goings, find out what you're up to." 26 Joab left David and went into action. He sent messengers after Abner; they caught up with him at the well at Sirah and brought him back. David knew nothing of all this. 27 When Abner got back to Hebron, Joab steered him aside at the gate for a personal word with him. There he stabbed him in the belly, killed him in cold blood for the murder of his brother Asahel. 28 Later on, when David heard what happened, he said, "Before God I and my kingdom are totally innocent of this murder of Abner son of Ner. 29 Joab and his entire family will always be under the curse of this bloodguilt. May they forever be victims of crippling diseases, violence, and famine." 30 (Joab and his brother, Abishai, murdered Abner because he had killed their brother Asahel at the battle of Gibeon.)
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.