Then said Abishai to David, God has delivered up your enemy into your hand this day: now therefore please let me strike him with the spear to the earth at one stroke, and I will not strike him the second time.
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."