Same place the art, the quadratic equation, infinity, morality and other abstract, transcendent ideas are: uniquely within human reason and conscience.
As a moral relativist, you, Stalin, and Hitler just have opinions.
The most you can say about Stalin and Hitler is that you disagree with them.
You cannot say they are objectively wrong, because you believe morals are relative, subjective, a creation of the popular vote. You cannot appeal to any higher objective moral standard to state Hitler is objectively wrong for all time and all places.