A fallacy in need of a name

At a party I attended earlier this year, one particularly vehement guest announced that, "Russia was justified in bombing civilian targets in Georgia because Georgia had killed ethnic Russian civilians in South Ossetia." I pointed out to the vehement fellow that neither Russian law, nor Georgian law, nor international law, nor moral reason allow for collective capital punishment based on ethnicity or nationality, and therefore of course it doesn't make any sense that if the Georgian military killed ethnic Russian civilians, the Russian military is allowed to kill Georgian civilians. Unable to refute this logic, he switched to another argument, using the same logical fallacy, claiming that we as Americans cannot object to the behavior of the Russian government, because the American government has killed so many civilians in so many countries. When I pointed out to him the possibility that the Russian and Georgian and American governments may all, simultaneously, be culpable for their own actions, and that we need not be apologists for any war crimes, despite the multiplicity of criminals, my wife suggested it was time we leave the party. This was a very sensible suggestion, so we did.

I've recently written about the importance of having names for common logical fallacies, and this got me wondering what the name for this guy's fallacy is. It is an extremely common one, and is in no way original to him. The fallacy is this: assuming or implying that if one side in a debate/argument/conflict is wrong/guilty the other side is therefore right/justified. This most often comes up in moral contexts, where the misdeeds of one party are used as a defense for the misdeed of the other party, but is also used in the context of disagreements about fact. For example when it comes out that an evolutionary biologists was wrong about anything, creationist make the leap to this being proof of Creation, rather than simply a flaw in the thinking of a biologist. One side is wrong, so the other must be right, even if the flaw is immaterial to the disagreement. Similarly, it is common in interpersonal disagreements to hear one party respond to an accusation of misbehavior only by pointing out a misbehavior of the other party without ever replying to the statement about their own behavior. This often goes well beyond "two wrongs make a right" to "my behavior is necessarily right because yours is wrong." This is also distinct (and almost opposite) from the idea of moral equivalence. In the Arab-Israeli conflict, for example, some people argue that the crimes committed by supporters of the two sides are morally equivalent, and therefore neither side can be held responsible. Each side often responds to this nonsense with the counterfallacy that our behavior, whatever it may be, is justified by their crimes. They are criminals therefore we are just. I would tend to assume that each party or individual is responsible for its own actions, and whether or not the crimes are equivalent is morally irrelevant.

So this raises three sets of questions for me:

1. What is this fallacy called? If we want to describe a response as following this pattern, how should we refer to the pattern?

2. Why do humans do this so freely? On some deep level are we programmed by evolution to use this type of rationalization? Is this culture specific, or do all humans do this?

3. Why is this particular type of illogic useful/successful in arguments? Perhaps it is particularly effective because it allows one to move from being on the defensive to being on the offensive, to accuse rather than excuse (or to effectively do both at once)? Or maybe it is just very hard for people to find fault in both sides of a conflict at once, so by making them see the fault in the other side you make them forget about the fault in your own?

I'd appreciate hearing your thoughts on this, particularly what this type of fallacy should be called, or if it already has a name. I've checked Wikipedia's list of fallacies, and it ain't there.