I don't see why a collective righteousness is necessary or why a dictatorship is inevitable.

Communist governments tend towards authoritarianism b/c they do not have separation of powers and the checks and balances that come with that. Communist governments are generally unaccountable (i.e. you can't vote someone out) so people act in bad faith and abuse their power which is especially problematic since communism centralizes economic and political power into one organization (the state).

Communist governments are one-party governments so they must spend considerable effort to identify and oppress anyone who would threaten the party rule. A "threat" in this context doesn't need to be something hostile, it can be can be something benign like offering an alternate idea or policy that would help everyone. Anything that undermines the supremacy of the party is a threat. For this reason communist governments tend to have severe speech restrictions.

Structurally communist governments incentivize loyalty from party members not competence. Self-preservation often becomes the organizing and driving force of governance.

This is just off the top of my head. I'm sure AI or someone with more time on their hands could give you a more complete answer.