I have to wonder whether Eco's approach is flawed in its systematizing.
It is a noble flaw in every intellectual to categorize, dissect and flay their prey. Then produce a general theory from the evidence scattered before him. Hayek used to bitch about it.
However another view could be the gaps that exist between Fascism and Communism are to be explained by historical circumstances, geography and the prevailing orthodoxies of the day rather than a grand theory of Anarchism, Capitalism, Communism or any other -ism.
Those could just be the retroactively generated cover stories. We like coherent stories to hang our ideas on. Notice for instance how World War II has gradually resolved in the public's mind as something like a historical fiction, involving 'good guys' and 'bad guys'. I am not a relativist but I find it frightening that 'good guys' has a flexible enough range to include Joesph Stalin.
Apart from the problem of treating a World War like a Cowboy Western, much less attention is paid today to the interaction between the extreme leftists (Bolsheviks and anarchists) and the extreme right (Nazis) in the lead up to World War II.
For instance I was very surprised to understand in the BBC documentary "The Nazis, a warning from history" (an excellent documentary) the degree to which the activities of the Bolsheviks influenced the rise of the Nazis. The hate directed towards the Jews is incomprehensible unless you grasp the fact that nearly every Bolshevik bomber was Jewish. My school textbook neglected to mention this and popular culture also uses racial hate as a deus ex machina of a backstory. It is a given, but never elaborated on.
This only finally clicked with me when I saw interviews with German POWs who were constantly harping on protecting civilization from the Bolsheviks. To them everything they did was justified in light of that. They weren't Nazis and they didn't appear to especially hate Jews. Still, they were part of the machine.
This means to me, that hate is a short term impulse and the actual destruction of minorities within Germany was in fact a clinical decision, fully rationalized and accepted by the public. The newspapers might have ranted but the public weren't composed mostly of bigots. They were composed of people who eventually just stopped caring because their way of life was under threat. The threat to a minority isn't necessarily hate, but a kind of widely shared exhaustion and apathy.
I fear those who complain we've forgotten our history, have actually forgotten our history for an absurd simplification about irrational hate. I feel like the mainstream interpretation of history is actually now the politically incorrect one.