I think we cannot think an absolute dichotomy between democracy and authoritarianism (or even totalitarianism). I would rather think a continuum between them.
Authoritarianism is defined by measuring the deviation from a democratic norm, which is not fixed. Therefore, we can find authoritarian elements in systems categorized as democratic (France, USA, etc) and democratic elements in systems categorized as authoritarian (even the Nazism). Also, all Soviet regimes are not as authoritarians, according to time and space.
Then, a socio-historical perspective would deny the absolute integration of nazism, fascism and communism in a same category of political system created during the Cold-War.
The use of theses terms is often too restrictive.