So I pretty much boiled down why I think empaths are afraid of sociopaths.
It's the fear of the unknown.
Let me explain.
When I was at the zoo a few years ago, I casually asked a zoo keeper why the giraffes don't choose to escape. They don't put up a fence high enough to prevent the giraffes from simply walking out of the enclosure. All there is a little ditch and a tiny wall. The zoo keeper explained to me that the reason the giraffes never leave is because they can't see what's in the ditch. Since they don't know what would happen if they tried to step over it, they choose to stay in their enclosure instead.
This is pretty good analogy for humans and death.
If God were to go “After life you get to be in heaven and it's perfect,” to everyone after they were born, people would throw themselves off of cliffs as soon as they were able to walk.
The reason people generally don't run around killing themselves is because they have no idea what happens after death.
So how does this relate to sociopaths?
People eventually had to come up with unspoken rules that would prevent people from hurting each other. Not because they cared about the other people, but because they didn't want to be killed themselves. They didn't want to face the unknown.
And thus we have “empathy” which is really just the fear of something that has happened to someone else either affecting you or happening to you.
What's really scary to an empath about sociopaths is that they have no idea what they're going to do next. Their thought process is completely foreign, since empaths have always functioned with, well, empathy.
Even if you explain how a sociopath thinks to an empath, they're still a little bit afraid. They have no idea what a sociopath will do, or maybe do to them, because they do not function under the normal silent rules humanity came up with. The sociopath's thought process is still relatively unknown to the empath.
So for an empath, trying to understand a sociopath is kind of like the giraffe trying to see what's in the ditch. You can kind of tell what's down there, but because you don't really know what, you'd prefer to stay away.
I hope that makes sense.
Empaths, see also this post for the answers to some of these questions.