2 Samuel 3:27-31

So when Abner returned to Hebron, Joab took him aside into the middle of the gate to speak with him privately, and there he struck him in the belly, so that he died on account of the blood of his brother Asahel.

Afterward, when David heard about this, he said, “I and my kingdom are innocent before the Lord forever of the blood of Abner the son of Ner.

May it turn upon the head of Joab and on all his father’s house; and may there not be eliminated from the house of Joab someone who suffers a discharge, or has leprosy, or holds the spindle, or falls by the sword, or lacks bread.”

So Joab and his brother Abishai killed Abner because he had put their brother Asahel to death in the battle at Gibeon. David Mourns Abner

Then David said to Joab and to all the people who were with him, “Tear your clothes and put on sackcloth, and mourn before Abner.” And King David walked behind the bier.