The other comment is definitely far too simplicistic in its proposition, but I’ll point out that Communism doesn’t have to be authoritarian. That’s just the result of violent revolution, necessarily carried out by people so convinced that their ideology is right that they’ll use violence to assert it. Revolution requires unity, so dissidents present a real risk to a nascent movement.

Combine those two and you have a recipe for authoritarian suppression of all who disagree with the dominant ideology, or the dominant leader figure supposedly best representing it. What they might initially see as a necessary step to a better world then becomes a feedback loop: Anyone who argues that they’re past the point where this policy is still necessary and justified is a dissident by definition.

Conversely, authoritarian policy also doesn’t require communism. It’s perfectly possible to have a non-communist ideology in power that suppresses all opposition. The problem isn’t communism, it’s violence: once started, it’s hard to reign in again and keep on the right track.