David prays not against Ahithophel's person, but against his counsel. He prayed this, in firm belief that God has all hearts in his hand, and tongues also. But we must second our prayers with endeavours, and David did so, else we tempt God.
But we do not find wisdom and simplicity so united in any mere man, that we can perceive nothing which needs forgiveness. Yet, when the Son of David was treated with all possible treachery and cruelty, his wisdom, meekness, candour, and patience, were perfect.
Him let us follow, cleave to, and serve, in life and in death.