In 1 Samuel 15:1-35 Samuel commands Saul to destroy the Amalekites. Saul destroys them, mostly. When confronted by Samuel he argues that his disobedience was really done to serve the Lord, since the things not destroyed were to be offered to God. Samuel then gives one of the great fundamental principles of service to God: Obedience is better than sacrifice. Attempting to do things for God that He has not commanded, that is making sacrifices for God in an attempt to serve Him in a way not found in the Bible, is actually a great sin. God desires obedience.