If we follow it backward hate comes from anger and anger comes from fear. And the strongest fear there is is the fear of the unknown.
Foreword. What I noticed among RL and fictional Fascists is that they A) IMAX project their failings on everyone not them because they cannot imagine other people being so dissimilar to themselves "so they must be hiding it!", and B) they actually cannot comprehend the inner mental goings of the Left because of completely different synaptic connections created during childhood. They really cannot, like "Puny Human mind cannot imagine Edltrich monster" not capable to grox the Left. AKA they are subconsciously scared shitless from the possibility that A) is not actually true and that they are dealing really with something incomprehensible to their world view.
Like in "Color Out of Space" the moment they would grasp it (or even get close to its vicinity) is the moment they would stop being themselves (fascists) and become something Alien. Capital A. Alien. Which results in them going PURGE! PURGE! PURGE! at all cost, at once, everywhere they spot Leftism.
One could argue that the core reason, underlying everything above it, for the existence of Fascism is based in an individual's obsession of defending one's ego's integrity at any cost. Emphasis 'any'.
I'm not convinced of there being a neurobiological explanation ("synaptic connections created during childhood") for being fascists.
This reminds me of the debate conducted in the late 1940s / 1950s / into the 1960s in sociology, with the group around Adorno and Frekel-Brunswik arguing for the existence of an "Authoritarian Personality", attributed to experiences of a violent and abusive upbringing, while the mainstream of both sociologists and psychologists have remained skeptical of the theory.
I do believe all political ideologies - and religions, too, and a number of other all-encompassing mental frames, too - have underpinnings in / connections to individual psychology, but individual psychology in turn is always a product of its social context. I think it is a dangerous and not very fruitful path to single out any of these ideologies and pathologise them on all possible levels.
Yes, adherents of fascism may have had, on average, a lower probability for high "tolerance of ambiguity". But they also had a higher probability of coming from middle class backgrounds, of having lived through WW1 etc. Fascism is as much a historical phenomenon as it is a class phenomenon and one that has to do with culture and psychology.
As much as I despise fascism, I don't think it's wise to pathologise its adherents. (Especially when we don't have an agreeable "therapy" or "prophylaxis" at hand.)