Anti-fascism is a big concern for many on the socialist left. Their definition of fascism can be quite elastic and always seems to exclude ultra-reactionary Islamists (who are often seen as an oppressed minority) but usually encompasses even those mildly concerned about immigration.
Not to impugn their sincerity, but many legitimately wonder whether the far-left are ignoring extremists in their own camp. As for micro-Marxist parties aspiring to overthrow every government in the world...if that isn't 'extremist', what is? We hear a lot about Golden Dawn in Greece, but almost nothing about the Stalinist KKE.
Even today, the memory of the communist totalitarian dictatorships (like the old Soviet Union, or modern Cuba) seem to be treated very mildly compared with the fascist totalitarian regimes, like Nazi Germany or Fascist Italy.