I don’t think human suffering should be allowed to have the commutative property; the suffering of one group is not negated by the suffering they visited upon others. The best we can do is to take heed from all suffering that we as a species are capable of terrible things.
Yet, it is natural that the suffering of the in-group is amplified, and its sins swept under the rug. In the most charitable characterization of this phenomena we can assume that the things not talked about are the things which bring us shame.
If I grasp your meaning you're saying that two wrongs don't make a right and in general I'd go along with that. The point I was trying to make, in my typically brash way, was that the American decision to drop nuclear bombs on Japan was a sort of a trolley problem proposition. The early capitulation caused by those bombs outweighed the loss of life as it would have killed less people than extending the war would. Whether you can fully morally justify this or not is moot.
The other prong of my argument is that Imperial Japan and its actions were the moral equivalent of Nazi Germany, although modern Germany at least acknowledges the war crimes, something that modern Japan seems very loathe to do.