The key difference is that even a good ideology would turn bad in a bad culture. That happens whether you talk about socialism or liberalism. I you looked at the reign of Maxmillien Robespierre in France, you could have argued that liberals are no different than Fascists. Liberals started the revolution in Iran and they ended up with Khomeini's islamic dictatorship. Again you could use that to argue that liberalism is the same as Fascism.

But why is this wrong? Because the outcomes of any violent revolution in countries with long traditions for oppression and dictatorship is messy. In a fight for life or death, the most democratic and freedomloving ideologue will not rise to the top. The most brutal pragmatist will rise to the top.

Communism is not all that different. Fascism in contrast is fundamentally different, because it typically happens in democracies or well developed countries which are then turned into oppressive dictatorships.

Communism isn't anything to fear at all, because communist dictatorships tend to be replacements for other oppressive dictatorships. One dictatorship replaced with another... who cares? What we care about is preserving democracy or developing it further. In this context Fascism is the more dangerous ideology. In democracies, socialists tend to be represented by social demcrats rather than communists. Communism really only has appeal where there is no democracy.