Authoritarian, or totalitarian tendencies are often naturally produced by absolute conviction in "us being good, them being evil". Even when one's ideology in theory insists on democracy, and compromise, it's very easy to tell "ok, we just need to save democracy/planet/nature/humanity by crushing the evil people, and then there will be bright future". This is compatible with practically any ideology, because it calls just for one exception, sort of war to end all wars. Besides, it's always possible to re-define the very meaning of democracy, and compromise to suit an urge to get rid of opponents.
Btw, your use of left-wing term is the classical No true Scotsman fallacy.
Btw, your use of left-wing term is the classical No true Scotsman fallacy.