It is only human for someone who ascribes wholly to liberalism (as they understand it) to also be convinced that the alternatives are fascism, nazism, and communism. After all, we project ourselves on the world out of necessity.
And they are not necessarily wrong in their belief about the alternatives.
But when that conviction is so prevalent and entrenched as to become a blind spot, then you have the curious phenomenon of people who think that anything different to their point of view (liberalism notwithstanding), no matter how slight, necessarily leads to the alternative extremes.
This is what happens when an idea is conflated with the person (or people) who ascribe to it or opine to — when they think they embody it and so what they say about it is true, indubitable, perfect and incapable of objection.
Ideas are always spoilt by people and ever it was thus.
Why? — Because ideas organise people and people crave power.