Anarchism

To add to this:

There's a fairly simple organic path to becoming a tankie (it's also the path to anarcho-authoritarianism) - some number of people, and particularly notably young and impressionable ones, don't invest the nevessary time and effort to fully wrap their heads around the necessary realities of a society truly without hierarchy and are too impatient to accept the fact that a stable classless society will almost certainly not come to be in the near future, and quite possibly not even within their lifetimes, and between the two, they just sort of jump to the conclusion that then the best (or even only possible) strategy is to claim the authority to institutionalize their desired system, completely failing to grasp the fact that institutionalizing it will immediately and permanently make it something other than what they claim to desire.

And from there on out, it's just cognitive dissonance and compensatory behavior.