Which actions did they take that were polar opposites?

USSR in 1918 granted equal women's rights through its constitution which was unheard of in liberal world where women were largely under legal guardianship of their husbands. Nazi Germany, meanwhile, actively suppressed them.

Bolsheviks abolished landlordism and redistributed land to peasantry as part of their revolutionary goal, Nazi Germany actively preserved large estates.

Not to mention the differences on who they oppressed (former exploiting classes vs workers + ethnic and racial groups), how they handled education, property, how they handled unions (having them spread communist thought instead of being independent organs vs actively dismantling them and enforcing collaboration between classes), etc.

You do talk about autocratic power a lot, and yes - if you look at it superficially then both countries were single party states. They were different though - in terms of its class character and the function they had. For instance:

• USSR's single party rule was (until bourgeois opportunism completely took over) a dictatorship of proletariat, meaning that the interests which the state advanced were that of the workers - the abolishment of private capital, land redistribution, development of productive forces to meet everyone's needs, etc. The suppression of dissent was also justified - immediate post-revolutionary periods are the most tumultuous, that's where you often get back to back revolutions, and this line of reasoning was justified historically with the Russian Civil War popping up shortly after. In other words, the power was used in an attempt to abolish capital, to achieve an entirely different mode of production.

• Nazi Germany's single party rule was there to preserve capitalism and the capitalist ruling class, suppression was used on political opponents to keep the monopoly on power, but also used on ethnic and racial groups so it certainly was more ideological rather than being a necessity at least in this regard.

People always say liberal democracy inevitably leads to fascism but is there actual evidence for this?

Not inevitably - if there's no worker militancy, then fascism is not necessary.

Still, as shown by historical materialist analysis of Capitalism and actual history (Germany, Italy), the system has internal contradictions that inevitably lead to crisis (falling rate of profit, overproduction, concentration of capital, bubbles - read Capital if you want an academic analysis on these), and if this capital under crisis also gets threatened by the workers, that's when you get fascism.

It's a good tool to overcome the existential crisis, suppress the worker militancy and commit some atrocities under the name of nationalism as the unifying cause.

Anyway yap yap wall of text nobody will read, these subjects tend to be much more complicated than "democracy good anything else bad and leads to hitlerism"