2 Samuel 3:31-33 And David said to Joab, and to all the people that were with him, Rend your clothes, and gird you with sackcloth, and mourn before Abner. And King David himself followed the bier. And they buried Abner in Hebron: and the king lifted up up his voice and wept at the grave of Abner; and all the people wept. And the king lamented over Abner.
After Saul’s death, there was civil war between the house of David and the house of Saul. At that time David’s general, Joab, took aside Saul’s general, Abner, and assassinated him. Abner was a sworn enemy of David’s and tried to prevent David from taking over as king. Joab (and we, too) thought he was doing David a favor, and surely he was surprised when David made such a public spectacle of weeping and mourning over Abner’s death.
David stood to gain personally from the death of Abner, but instead he acted as if his best friend had died. There was probably some political favor to be gained from public mourning and weeping. But more likely his spirit was simply grieved at Sin in general and the death and destruction caused by it. He did not rejoice that he now had the political advantage, but he mourned at the loss of human life and the sinful condition of humanity. It was not selfish grief since it was gain for him and not loss. Believe it or no