Hi Clay,
My experience is that fear in young autistics is often caused by mutual incomprehension which can manifest in many ways, including outright rejection.
But because of enormous advances in early intervention, which educates the parents and sibs as much as their autistic family member, fear is not necessarily even a consideration among autistic infants today, although it's likely to surface in adolescence among those of us able to function in the community.
Those of us not so able more likely feel frustration; their lives are quite often dominated by a low-level anger which can flare very quickly.
OK, I've had my say. :-)
I was once asked whether or not there had been any quality, anything at all that I remembered above all else from my childhood. It took me at least a minute to formulate an answer which was "fear".
I believe that fear is a constant in the majority of autistic childhoods but at no time have I said that it's the "primary emotion".