You shall not kill.
Nobody can argue with the commandment against murder. Murder is bad. Everybody knows this. So why command something that everybody already knows and accepts? Because we only know it sporadically. Situations arise in which we have to be reminded that what we know to be true and right in happy moments continues to be true and right even when a fit of rage is on us. In short, commandments are given in order to reinforce minimal moral requirements in the face of temporary assaults on reason. They show us the bottom-most limits of morality and virtue: if you can’t love your neighbor, at least don’t beat his head in with a baseball bat. It’s important to remember that these bottom limits are merely the bottom, not the heights, of what we are called to in Christ. Merely not killing somebody is not exactly a glittering example of the splendor and holiness of God’s love, so boasting that we are “good enough” simply because we observe minimal morality is insufficient. We still need a Savior. On the other hand, under the power of grace, minimal morality is a starting place in those desperate moments when we are really tempted to murder that jerk at work that has abused us for years. And since God is pleased with our faltering efforts as much as with the great deeds of giants like St. Paul, He can turn the widow’s mite of our struggles with anger into a great spiritual fortune for His glory.