I've recently discovered that an argument I've often seen and sometimes used is based on something untrue. Christians pacifists (and pacifists intending to win over Christians) often make the claim that, since one of the ten commandments says "do not kill", it must always be immoral to kill. I've also seen the sixth commandment come up in lists of supposed Bible contradictions. Most such lists are filled with mainly easily-resolved surface-language differences with the occasional serious difficulty that takes some real work to resolve (although I know of no such difficulties that don't have at least one possible solution, thus showing that it's not actually a contradiction).
One (among several) responses to both of these claims is that the word used for murder in the sixth commandment in fact does not mean killing but simply means murder, so the only kinds of killing that it could be talking about are those that are wrong, leaving it open that there are kinds of killing that are not wrong. It turns out that this isn't true. There are several words for killing in biblical Hebrew, and this term isn't the most common one. It's usually reserved for contexts of killing within the covenant community, usually used in cases where the killing is especially divisive, often with inter-tribal conflicts in mind.
Its most frequent occurrences are all in one chapter, though, and that chapter is Numbers 25, which provides the details of the city of refuge provision of the Mosaic law. The ancient near eastern method of bringing murders to justice was to have an appointed avenger within each extended family or clan unit, who would hunt down and kill anyone who killed one of their own. The city of refuge provision took several of the Levitical cities and made them safe havens from avengers until a trial could take place, thus ensuring justice could be pursued more carefully as long as the accused was willing to flee to one of those cities. If the person was not found guilty of deliberate murder, they could live in the Levitical city until the death of the current high priest atoned for their sin of negligence, but otherwise they could be put to death once convicted.
I don't remember all the details now, but after looking over this with someone who knows Hebrew I discovered that most or all of the occurrences of deliberate murder used the same word as in the sixth commandment, but the term also occurs two or three times of the killing by the avenger, which as far as I can determine is legally sanctioned killing. It's not used of outright death penalties for specific crimes in the Torah, but it is used of the avenger's killing of duly convicted criminals. So what was probably the easiest response to the difficulties I mentioned above doesn't seem to be correct. The pacifist may not be able to claim that what the commandment says not to do can cover every kind of killing, but they can claim that the word can be used for legalized killing. Also, you can't get out of the supposed contradiction simply by saying the word doesn't mean "kill" but means "murder", since the Torah seems to allow instances of killing that use this very word. But I don't think this puts a stop to the kind of view I would defend. It just makes one of the easier and quicker responses no longer as easy and quick as I would have liked.
1. The flavor of the term used for killing in Exodus 20:13 is killing within the covenant community. So even if it forbids all killing within the covenant community of Israel, the sixth commandment can't be used by itself to justify universal pacifism. At most it could support pacifism with respect to those within the covenant. The pacifist would have to appeal to Genesis 9 for a more universal prohibition on killing.
2. Genesis 9 includes the first imposition of the death penalty, which is treated as a universal moral principle. The pacifist has a hard time prohibiting all killing when the very passage that seems most to justify it also implements a death penalty for those who violate it.
3. There are places in the Torah that Christians don't observe, most especially when the New Testament itself explicitly removes such practices. The Sabbath, for instance, is fulfilled in Christ, and it's clear in the New Testament that observing special days is in the same category as circumcision -- at best optional and quite possibly counterproductive if it leads to a replacement of the gospel with legalism. The New Testament in this case doesn't remove capital punishment, however. In fact, Romans 13 explicitly includes the use of the sword for promoting justice as one of the legitimate functions of God's appointment of leaders serving in government.
That doesn't deal with the contradiction charge, since it could very well still be that Genesis 9 contradicts itself, just as the death penalties and holy war passages in the Mosaic law might contradict Genesis 9 (and the death penalties might contradict the sixth commandment) for all I've said so far. So more needs to be said for that.
3. One thing to keep in mind is that just because a word can mean something doesn't mean it always includes that sense in every context. So it doesn't follow from the fact that this word can be used of legitimate killing that the prohibition on killing includes a prohibition on legalized killing.
4. In fact, the very existence of legalized killing in the same law code as the sixth commandment is strong evidence that it wasn't intended to cover that category of killing. It's often perfectly legitimate to speak of something that, when taken literally, is false but when taken in a restrictive context is perfectly fine. Saying there's no milk left in the fridge in order to justify buying more is perfectly ok even if there's a tiny puddle of milk at the bottom of the vegetable crisper. Some might object if they hold to a piecemeal approach to the composition of the Pentateuch according to which materials from different time periods and perspectives were combined haphazardly.
But such a view is at odds with the internal evidence of those documents. It's highly implausible that editors would put together contradictory materials in the same passage and not make any attempt to resolve the contradiction, and Genesis 9 includes both the prohibition on general killing along with the death penalty if it's violated. I know of no theory of composition that places these with different sources, but such a view would be on the face of it implausible if it attributed to the editor the utter stupidity that would require putting two contradictory statements immediately next to each other without recognizing the incongruity.
So the best way to take prohibitions on killing is in a more restricted sense in some way, even if the same word can in other places refer to legitimate killing.
5. A view that seems highly plausible to me is that the Genesis 9 passages teaches the general moral badness of killing, emphasizing its seriousness without explaining the moral complexities in practice that the rest of the Torah sometimes will detail with various nuances. It merely gives the most obvious exception, which is that the moral seriousness of murder is so great that one loses one's own right to continue living. Then Exodus 20:13 presents the application of the principle among members of the covenant with the same generality, also without offering the various nuances that the rest of the Torah details. Then the various instances when the presumption against killing is lifted will give care to explaining which exact circumstances those will be, and the various instances when the penalty for killing is mitigated or removed will also explain the details of how that works. This is a perfectly coherent view that much better explains the text as it stands, without resorting to claiming contradiction but without denying the serious nature of the original moral principle behind killing being always bad.
So I think there's plenty to say against both the absolute pacifist's use of the sixth commandment and the claims of those who would classify this surface contradiction as a genuine contradiction, all without making the simpler argument that turns out to rely on a false view of the word for killing in Genesis 20:13. It just takes a little more work to think through the issues and come to a reasonable reconstruction of what's going on in the various texts. But that's true of most biblical interpretation and indeed most interpretation of texts arising from cultural environments somewhat foreign to us. I don't think there's anything especially difficult about this case. I just needed to fill in the details, since I have used the faulty argument myself in the past.