Now, you may take those words at face value, and if you do, their intent is pretty obvious. Killing, it would seem, is wrong.
Mind you, there is not a lot of specificity there. Kill what, precisely? Other people? Animals? Microbes?
Without context, as powerful as the words are, we are left to wonder about their true intent. When placed in the context from which they sprang, the Ten Commandments, there is much better clarity. The Ten Commandments were a series of rules to live by, the codifying of life in terms all could understand and recognize. They told you what to do and what not to do to be seen as good in the eyes of the Old Testament God.
And these 4 words makes sense, even out of that context.
Whatever your belief system, there seems to be a fundamental right to be alive. You are here, brought into this world, raised, part of human society, and it seems perfectly natural for you to stay that way. More to the point, given that we are all human in this society, it seems reasonable to assume that if you have the right to be alive, so do your fellow humans. Taken further, it is an unforgivable breach for one human being to deprive another human being of their life, an asset that cannot be restored, once taken.
What seems obvious, right, and logical to one person, or even group of people, does not always translate. Throughout time, it has always been easier to simply deprive others of their life in order to get what you want. Someone has land you want: you kill them. Someone is in love with a woman you desire: you kill them. Your country is in a shambles and those people are to blame: let us kill them. They do not look like we do: kill them. They do not believe what we do.
Miraculously, human society still exists, despite the ever-increasing conflicts we pursue, on the ever-increasing scales we pursue them. And where large portions of human society have not managed to wipe other large portions out, there are the small, daily skirmishes, chalked up to daily survival, or worse, the defense of a "way of life."
You shall not kill.
For all the power of those 4 words, they are easily enough ignored when desired. Even the most devout can find a justification for murdering someone, if they try hard enough. Perhaps it is easy to do, because they know they can ask for absolution, a kind of "get out of jail free card" for the soul. Perhaps they are convinced that it is the hand of God that made them do it, and that places them beyond the grip of mortal, human justice. Perhaps they need no more justification than "I can do it, so I will." They do not fear degradation, retribution, or the judgment of other humans, cocooned in a shell of their belief, as unbreakable as their personal faith.
So, it is easy enough for them to justify their actions to themselves, if no one else. Yet we know, there are those who believe as they do, who see these actions, not as deplorable or reprehensible, but as justified, on some loose moral ground. They are driven by a desire to see everyone believe as they do. They are frustrated that their will is not enforced. They see it as their "right" to make the world in their own image. Reason, appeals to sanity, logic... these do not sway them. Their belief is their sword and their shield, and backed by nothing more, they would trample others who stand in their way, all the while hiding behind the protections granted them by their country.
Death begets death. They who live by the sword, die by the sword.
And those who would take up arms to enforce their point of view, or revel in a blow struck for their beliefs, can rest peacefully in their bed, because nowhere will others who oppose them plan the same fate for them. They will not have to worry about being brutally murdered to prove a point.
Violence is the last refuge of the incompetent.