When it comes to WW2 era and the millions of innocent civilians killed under authoritative fascist and communist regimes, people often want to separate all the different players such as National Socialist Germany and Communist Soviet Russia. Certainly there are differences between them. But in people arguing whether National Socialist Germany was right-wing Fascist, or if Communist Soviet Russia was truly left-wing, I think a more obvious, and less ambiguous, point is missed. All of the authoritative regimes were anti-Capitalist, they were anti-Free-Market.
Anti-Capitalism was not a surface level point among the violent authoritative regimes in WW2, it was a core piece. From Hitler to Stalin, they ran on a very misguided notion that Capitalism is fundamentally, and unavoidably a system by which a so called 'Bourgeoisie' elite class exploits a 'Proletariat' worker class. Hitler made a strict association of Capitalism being a creation of Jewish people. This is the often missed point of National Socialist Germany. The holocaust was not so much based on hating Jewish religious tradition as it was based on Hitler's belief the Jewish people created Capitalism and Capitalism was the source of the destruction of the German people. Hitler saw the Jewish population as a socioeconomic class who represented the detested Capitalist and bourgeoisie caste. This aspect of National Socialist Germany is more thoroughly expounded, and cited, on Wikipedia:
https://en.wikipedia.org/wiki/Nazism#Anti-capitalism
Conversely, and obviously, Communist Soviet Russia was anti-Capitalist, as most people are quite aware of the dichotomy between Capitalist and Communist governments. But I think it's important to recognize all authoritative regimes of WW2 believed they were fundamentally fighting to end class struggle that they believed was wrong and unjustly created by Capitalism.
History repeats itself, an often repeated phrase in political activism these days. But most people miss what aspect they actually need to watch out for. It is not race and religion. Rather it is the attempt to demonize and tear down Free Market Capitalism. It is the notion that a Free and uncontrolled economy exploits people and must be controlled by government in order to avoid this.
In this way, there is no difference between radical left and radical right of the modern world. Yes, obviously there is a difference in full comparison, but there isn't a difference that matters. We don't want any repeat of the notion that government authority is the solution to removal of socioeconomic classes created by Capitalism.