And David commanded his young men, and they killed them and cut off their hands and feet and hanged them beside the pool at Hebron. But they took the head of Ish-bosheth and buried it in the tomb of Abner at Hebron.
We have here justice done upon the murderers of Ish-bosheth. I. Sentence passed upon them. There needed no evidence, their own tongues witnessed against them; they were so far from denying the fact that they gloried in it. David therefore shows them the heinousness of the crime, and that blood called for blood from his hand, who was now the chief magistrate, and was by office the avenger of blood.
Commenting on 2 Samuel 4:9-12
DAVID CAUSES THEM TO BE PUT TO DEATH. (Sa2 4:10-12) slew them, and cut off their hands and their feet--as the instruments in perpetrating their crime. The exposure of the mutilated remains was intended as not only a punishment of their crime, but also the attestation of David's abhorrence. Next: 2 Samuel Chapter 5
And they slew them - None ever more richly deserved death; and by this act of justice, David showed to all Israel that he was a decided enemy to the destruction of Saul's family; and that none could lift up their hands against any of them without meeting with condign punishment.