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.
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.
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.
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 1 or who walks on crutches 2 or dies by the sword or begs for food!”
So Joab and his brother Abishai killed Abner because Abner had killed their brother Asahel at the battle of Gibeon.