David arbitrarily killed 2/3 of the prisoners of war in 
>NIV 2 Samuel 8:2 
David also defeated the Moabites. He made them lie down on the ground and measured them off with a length of cord. Every two lengths of them were put to death, and the **third length was allowed to live**. So the Moabites became subject to David and brought him tribute.

The he hamstrung almost all the horses in

>NIV 2 Samuel 8:4 
David captured a thousand of his chariots, seven thousand charioteers and twenty thousand foot soldiers. He **hamstrung** all but a hundred of the chariot horses.

To hamstring a horse and leave it crippled is terribly cruel. If he had killed all the prisoners and horses, that would make more sense. Why the arbitrariness? Why the cruel treatment of horses? What's to be accomplished by these cruel acts that he could not accomplish without them?