Q: Which is the sixth commandment?
A: The sixth commandment is, You shall not murder.
Romans 13:9 For the commandments, “You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet,” and any other commandment, are summed up in this word: “You shall love your neighbor as yourself.”
This is one of those commandments where many of us think, "this is an easy one to keep!" But remember, the commandments are summaries of God's law. There's much more to keeping any one of the commandments than simply obeying the outward sign. So it is with this commandment. There is much more to keeping this commandment than simply not taking another person's life. There's a heart issue at play. This is why Jesus says on the sermon on the mount in Matthew 5, "You have heard that it was said to those of old, ‘You shall not murder; and whoever murders will be liable to judgment.’ But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, ‘You fool!’ will be liable to the hell of fire."
The Heidelberg Catechism has a wonderful section on the 6th commandment, showing us how it is not only the outward acts that matter, but also the inward heart reality:
106. Q. But does this commandment speak only of killing?
A. By forbidding murder God teaches us that he hates the root of murder, such as envy, hatred, anger, and desire of revenge, and that he regards all these as murder.
107. Q. Is it enough, then that we do not kill our neighbor in any such way?
A. No. When God condemns envy, hatred, and anger, he commands us to love our neighbor as ourselves, to show patience, peace, gentleness, mercy, and friendliness toward him, to protect him from harm as much as we can, and to do good even to our enemies.