"Thou shalt not bear false witness against thy neighbor," the eighth commandment (in some numberings, ninth) reads.
When I was growing up in Sunday school our teachers usually paraphrased the archaic King James Version English of that commandment as "don't lie." Teaching children not to lie is a good Sunday school lesson, but note that this isn't actually what the commandment says. It's much more specific, prohibiting a particular kind of lying — "false witness." A better children's paraphrase might be "don't accuse anyone of something they didn't do" or "don't make up bad things about other people."
The distinction and the specificity matters. Lying is a bad thing and if you're teaching small children in Sunday school about the importance of telling the truth, there are plenty of other Bible passages you can cite to make that point. But this particular kind of lying — bearing false witness — is singled out as particularly bad. It's corrosive and enslaving in a way that other lying may not always be.
To explore this, I'd like to revisit a classic Ethics 101 hypothetical situation involving, as so many of these hypothetical situations seem to, Nazis. (I apologize for violating the "Godwin" convention.)
Say you're living in occupied Holland during World War II and you've got a neighboring Jewish family hidden in your attic. A local busybody, a collaborator eager to curry favor with the occupying Nazi government, comes sniffing around looking for anything he might learn that would earn the oppressor's praise. Is it acceptable to lie to this man, to deceive him in order to ensure the safety of the innocent family you are helping to rescue?
Note that this classic hypothetical dilemma was not-at-all hypothetical for many actual people who lived it. The righteous gentiles of the Netherlands — people like the Ten Boom family or the many helpers who tried to save Anne Frank's family — were constantly confronted by this very real, high-stakes situation. And every time, they lied. They actively, aggressively worked to deceive the collaborators and the Nazis themselves, lying, misleading, forging papers and deceiving without hesitation or remorse.
I believe they were right to do so. I think this is obvious and uncontroversial. This is, in fact, the judgment of history. These people are remembered as righteous gentiles, after all, because they chose to lie to protect the innocent.
There are schools of thought which regard the moral duty never to lie as applying even in cases such as this. (Michael Sandel has an engaging discussion of Kant's views on this, if you're interested. As far as that consequentialist/inconsequentialist argument goes, I'll see your Kant and raise you a Bonhoeffer.) My point here is not to rehash that argument, but simply to point out that this sort of lie — deceiving an evildoer to protect the innocent from harm — is a wholly different species from the sort of lie prohibited in the Ten Commandments. The rescuers lied, but they did not bear false witness against their neighbors.
Lying about others — bearing false witness against them — is dangerously corrosive. It sets the liar on a downward path that leads not just to moral confusion, but to epistemological insanity. Bearing false witness will ultimately make you crazy.
What may start out as a well-intentioned choice to "fight dirty" for a righteous cause gradually forces the bearers of false witness to behave as though their false testimony were true. This is treacherous — behaving in accord with unreality is never effective, wise or safe. Ultimately, the bearers of false witness come to believe their own lies. They come to be trapped in their own fantasy world, no longer willing or able to separate reality from unreality. Once the bearers of false witness are that far gone it may be too late to set them free from their self-constructed prisons.
This slide from fighting dirty to embracing insanity happens in politics, obviously, but not only in politics. And regardless of the arena the end result is the same. The bearers of false witness make themselves stupid — so stupid that they don't even seem to notice that they've surrendered the argument by choosing to live in a fantasy world in which all arguments are irrelevant.
Anyway, contra Kant, I believe it may be justified and just sometimes to lie to evildoers. But don't lie about others, not even about those you regard as evildoers. That's never justified and it won't end well for you.