What leads you to believe that these regimes came to power "peacefully"? That most of them did so without a protected civil war as in Spain does not imply that they were bloodless.These regimes were in crisis, and the elite turn to authoritarian rule, fascist or otherwise, represented a last ditch attempt to preserve their power. Weimar Germany would not have been magically transformed into a healthy liberal democracy if the Communists decided to abide by liberal norms. And we know this, because Communists in Spain, France and elsewhere did this during the "Popular Front" period, and fascism or elite support for fascism didn't simply dissipate.
Weimar Germany might have had more chance if the Communists hadn't spent the 20s attacking the socialists rather than the fascists. Their conversion to the Popular Front came too late and they weren't trusted by those they belatedly tried to ally.