1 Samuel 15

9: God's logic is not the same as our logic. It seems logical to do what Saul did. But God, seeing the bigger picture, knew different. We should never think that we know what's better for our wellfair than God does. It seemed logical to spare the livestock, but it was not what God wanted. We must never allow judgment to supersede to God.  

15: Why does Saul say "the Lord YOUR God"?

quite the imagery, Samuel hacking another man to death.