It is often the simplest verses that cause the most confusion, and Exodus 20:13 is a prime example. Christianity is a notoriously peaceful religion that famously condemns killing. Or does it? Those who begin their studies are often shocked to learn that there are scenarios in which killing is not only permitted, but in many scenarios, directly commanded by God Himself.
The main source of confusion is the limitations of translation. Different Bibles have different renderings of Exodus 20:13. The HCSB says "Do not murder." The KJV, most famously, says "Thou shalt not kill". The NKJV, which is significantly descended from the KJV, says "You shall not murder." Of all the Bibles I compare in my regular study (13 total), the majority rendering is "murder". Wycliffe's Bible is an anomaly, rendering it "Thou shalt not slay." Only the KJV, NABRE and Douay-Rheims translations use "kill".
There is a reason for this. The word "kill" is a very broad term, encompassing any termination of biological life. You can even be said to have "killed" a plant, even though plants aren't really alive in the same sense as humans and animals. Murder is not such a broad term. It refers to a very specific type of killing. It is specifically the intentional termination of a human life without a just cause. Accidental killing is not murder, because the intention is completely absent. Killing in self defence is not murder, because aside from the fact the intention is usually different, the cause is just. Capital punishment for high crimes is not murder, because the intention is just. Of course, capital punishment also has the added layer of requiring authority. A citizen cannot carry out a judicial execution simply because they do not have that authority.
It is significant that it tends to be the older translations which say "kill". Put simply, kill was the best term available at the time. Anyone living at the time would have understood it the same way we understand "you shall not murder".
And so we see that this remarkably simple verse really is remarkably simple. The confusion, while understandable, is nevertheless quite easy to clear up. Yes, killing is sometimes permissible in Christianity, but it is obviously far from ideal. The overwhelming majority of civilians should be able to live a long and happy life with a whopping body count of zero. If you're unfortunate enough to be an exception, ensure that it is because you had no other choice. Even if you are a police officer, or a soldier, or some profession where taking a human life is a distinct possibility, the default position should be to preserve life, not to run around firing indiscriminately into a crowd. On the whole, human life is extremely precious, and not to be taken lightly.