This thing isn't good that you have done. As Yahweh lives, you are worthy to die, because you have not kept watch over your lord, Yahweh's anointed. Now see where the king's spear is, and the jar of water that was at his head.
So David is sort of chiding Abner the chief general of Saul, saying, "Hey you know your job is to guard the king, and you aren't doing a good job. You ought to be put to death. Someone came in to destroy the king, and you were just sound asleep. Look where the king's spear is, and the cruse of water."