Why did God accept David who killed Uriah and took his wife but rejected Saul who spared King Agag?

• A David humbled himself when he realized his sin but Saul was stubborn
• B David was a singer of praises while Saul was not
• C David was a man after God's heart while Saul was His enemy
• D David was from the favoured tribe of Judah while Saul was from Benjamin