I don't know man, i think you should google night terrors or sleep paralysis. I dont know much about the worst inherent fear in humans, i guess it would be death, or maybe pain... but maybe its more than that. Maybe when introduced to ideas of demons this automatically gets put into our brains as (most feared thing) for us. As demons are the only "evil" beings we represent with surviving death, or existing after it, or meeting us there. Already dead beings, or devils... as everyone who has ever experienced night terrors can empathize with the unmovable unshakable intense feeling of dread and evil. The inability to talk or move anything but your eyes.
I really cant say what it is, personally... i think the similarities have everything to do with the most feared idea that humans have, or brings out the fears that we have the most, if the latter is the case, there must be many MANY out there who consider the idea of evil "dead" beings our subconscious's worst fear.
Here's a copy:
Over the centuries, symptoms of sleep paralysis have been described in many ways and often attributed to an "evil" presence: unseen night demons in ancient times, the old hag in Shakespeare's Romeo and Juliet, and alien abductors. Almost every culture throughout history has had stories of shadowy evil creatures that terrify helpless humans at night. People have long sought explanations for this mysterious sleep-time paralysis and the accompanying feelings of terror.