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You wrote: "Would you have permitted the torture of one person if it would have prevented the Holocaust?"
If per impossible such a thing would happen, I would argue that in the very moment your magical action prevented the Holocaust, you would have laid the inevitable root for a new one to spring up, by becoming what you thought you had destroyed.
If what you call a first principle admits of an exception, then it is not a first principle. You are saying you have no first principles. That you would degrade another human being in certain circumstances. I think that thought is the first step of a descent into moral madness.