Communism wouldn't have existed if the Catholic Kings had crushed the progenitors of the Reformation the same way they crushed the Cathars.

Actually communism only seems to have taken hold in monarchies and other right-wing tyrannies that attempted to stifle political speech. Countries like Russia and China and Cuba, as opposed to countries like England and France and the United States, or even countries like Canada and Sweden. Even countries recently threatened to fall under communist control were authoritarian monarchies like Nepal.

Not very paradoxically the power of communist regimes to slaughter and oppress tends to derive from the authoritarian mechanisms of the state they inherited.

So I think you have it the other way around.