Authoritarianism can come from any direction. You are absolutely correct in your diagnosis of Chavez and Castro. We went through a period where the greatest threats to liberal democracy were from the left. Populists used socialism to make promises to the people while they were gobbling up wealth and power for themselves
But what you need to realize is that the threat is Authoritarianism more broadly. Authoritarians crave power. They could care less about what economic system or brand of populism they use to collect that power. They will use whatever tools that most effectively retain that power, which are going to vary culture to culture and based on what's happening at that moment in time. Left or right wing Authoritarians follow the same basic playbook - undermine the free press - systematically attack and weaken institutions and norms that push back - demonize or dehumanize the opposition - etc. Right/left is irrelevant when it comes to that stuff.
Look at Erdogan in Turkey or
Orban in Hungary. We're watching right-wing authoritarian rule develop in real time. The tides have shifted. Just as lefties in the US sympathized with Chavez and Castro,
we are watching right-wingers here sympathize with Orban and Erdogan.
If you truly believe in the tenets of western liberal democracy, then you oppose threats from both ends. Bernie's brand of "socialism" is FAR less threatening to the principles of democracy than Trump's brand of politics. Trump is literally
following the authoritarian playbook. Scholars who have studied authoritarian rule for
decades are telling us how serious this is.
If Sanders is elected and spends his days intimidating witnesses on twitter, publicly telling his AG how to handle cases involving his allies, demonizing the free press, leading chants to jail his political enemies, crushing dissent in his own party, demanding loyalty to himself rather than the constitution - then I'll 100% be as worried about Sanders as I am about Trump. You're concerned with hypothetical threats of left-wing authoritarianism while
right-wing authoritarianism is festering in broad daylight right under your nose.