25 You know perfectly well that he came to spy on you and find out everything you're doing!" 26 Joab then left David and sent messengers to catch up with Abner, asking him to return. They found him at the well of Sirah and brought him back, though David knew nothing about it. 27 When Abner arrived back at Hebron, Joab took him aside at the gateway as if to speak with him privately. But then he stabbed Abner in the stomach and killed him in revenge for killing his brother Asahel. 28 When David heard about it, he declared, "I vow by the Lord that I and my kingdom are forever innocent of this crime against Abner son of Ner. 29 Joab and his family are the guilty ones. May the family of Joab be cursed in every generation with a man who has open sores or leprosy or who walks on crutches or dies by the sword or begs for food!" 30 So Joab and his brother Abishai killed Abner because Abner 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.