Ambrose of Milan : Although he [Saul] was a king, he sinned if he killed the innocent. Finally, even David, when he was in possession of his kingdom and had heard that an innocent man named Abner was slain by Joab, the leader of his army, said, "I and my kingdom are innocent now and forever of the blood of Abner the son of Ner," and he fasted in sorrow.These things I have written not to disconcert you but that the example of kings may stir you to remove this sin from your kingdom, for you will remove it by humbling your soul before God. You are a man, you have met temptation—conquer it. Sin is not removed except by tears and penance. No angel or archangel can remove it; it is God himself who alone can say, "I am with you"; if we have sinned, he does not forgive us unless we do penance. — LETTER 51.10-11