When David heard about this later, he said, "I and my kingdom are forever innocent before the LORD concerning the shedding of the blood of Abner, Ner's son.
May it fall upon the head of Joab and his entire family tree! May Joab's family never be without someone with a discharge or a skin disease, someone who uses a crutch, someone who dies by the sword, or someone who is hungry!"
References for 2 Samuel 3:29
So that is how Joab and his brother Abishai murdered Abner, because he killed their brother Asahel in the battle at Gibeon.
Then David ordered Joab and all the troops who were with him, "Tear your clothes and put on funeral clothes! Mourn for Abner!" King David himself walked behind the body.
They buried Abner in Hebron. The king wept loudly at Abner's grave. All the troops cried too.
Then the king sang this funeral song for Abner: "Should Abner have died like a fool dies?
References for 2 Samuel 3:33
Your hands weren't bound, your feet weren't chained, but you have fallen like someone falls before the wicked." Then the troops cried over Abner again.
Then all the soldiers came to urge David to eat something while it was still day, but David swore, "May God deal harshly with me and worse still if I eat bread or anything else before the sun goes down."
All the troops took notice of this and were pleased by it. Indeed, everything that the king did pleased them.
So on that day all the troops and all Israel knew that it wasn't the king's idea to kill Abner, Ner's son.
The king told his soldiers, "Don't you know that a prince and a great man in Israel has fallen today?
And today, though I am the anointed king, I am weak. These men, Zeruiah's sons, are too strong for me. May the LORD repay the one who does evil according to the evil they did!"
References for 2 Samuel 3:39