Let's say that Countries A, B, C, and D defeated Country E and wanted to place its soldiers and officials on trial for violating existing laws. If Country E was a rump-State, the only impartial way to do this would be to convene a court comprised of neutral countries F, G, H, and I (of equal international standing, more or less) in which case they would have to use standards of conduct applicable in theory also to the behavior of the victorious Countries A, B, C, and D.
Thus the Defense (hosted by neutrals and not by vulnerable citizens of rump-Country E at the pleasure of the Victors) would certainly use tu quoque
arguments where appropriate.
Of course, the neutral court could not charge citizens of the victorious sovereign countries A, B, C, and D, however, because they have their own competent jurisdictions.
And the neutral courts could not generate ex post facto
laws or (in theory) political accusations, as the Victors would be wont.
Most of the Nuremberg trials were not about Warcrimes or crimes against existing laws, but about political accusations such as Aggressive War, Conspiracy Against Peace, and Crimes Against Humanity (whatever that means).
Basically, Nuremberg was a propaganda fraud asserted by the force of the Victors.
What could have been done--though it would not have satisfied the true objective of Allied propaganda--would have been to convene a neutral court representing nations such as Sweden, Switzerland, Turkey, and Spain, and the victorious Allies could present their evidence of the violation of existing laws (i.e., crimes of war and not political-crimes) by a rump-State's citizens and soldiers in Allied custody.
Of course, Sovereigns like Hitler and Göring could hardly be accused of the political-crimes tossed at Nuremberg, and Allied soldiers and officials would be publicly humiliated (by the Defense) for their own conduct of warcrimes. And that just wouldn't serve the interests of Allied propaganda, which needed to discredit the German military and political leaders to better cow the German people--who would resist a blanket War-Guilt claim, as with Versailles.
A show-trial was also needed to justify the Allied Peace and for catharsis for both the Germans and the Allied public for the end of a long war with much deprivation and suffering on both sides. It had to appear like JUSTICE, and the best legal hacks of the Anglo-Saxon nations were therefore employed (in spite of Soviet participation in the process).
That's why Churchill's idea to convene mock trials was so brilliant over Stalin's equally-valid exemplary retribution, which would have undoubtedly created martyrs.
However, Nuremberg set Justice back to the Middle Ages by employing it in Soviet-style for political purposes.