When one reads the Larger Catechism, Q. 135, one finds a lilliputian world of commands contained within the one small seed of "thou shalt not kill." Here is the catechism:
Q. 135. What are the duties required in the sixth commandment?One of the questions we are going to be asking this coming week is, what on earth is our catechism doing talking about sleep, cheerfulness, forgiveness, and charitable thoughts in a commandment that simply says, "Thou shalt not kill"?
A. The duties required in the sixth commandment are all careful studies, and lawful endeavors, to preserve the life of ourselves and others by resisting all thoughts and purposes, subduing all passions, and avoiding all occasions, temptations, and practices, which tend to the unjust taking away the life of any; by just defence thereof against violence, patient bearing of the hand of God, quietness of mind, cheerfulness of spirit; a sober use of meat, drink, physic, sleep, labour, and recreations; by charitable thoughts, love, compassion, meekness, gentleness, kindness; peaceable, mild and courteous speeches and behaviour; forbearance, readiness to be reconciled, patient bearing and forgiving of injuries, and requiting good for evil; comforting and succouring the distressed and protecting and defending the innocent.
I like what Charles Hodge says about this, because it's quite helpful in seeing how the layers of this onion get peeled back in the first place: "In the several commandments of the decalogue, the highest manifestation of any evil is selected for prohibition, with the intention of including all lesser forms of the same evil. In forbidding murder, all degrees and manifestations of malicious feeling are forbidden" (3.19.10).
This is, of course, confirmed in the New Testament when Jesus in Matt. 5:21-22 directly connects the command not to murder with a seemingly lesser command not to be angry with or to insult one's brother.
We don't normally think of our anger in traffic, malicious feelings toward this or that co-worker, or despising someone who publicly disagrees with us about something as being a violation of the sixth commandment - and yet that is precisely what it is. We may not all be killers, but we are all nonetheless be guilty of violating the sixth commandment in one way or another.