Monsters. They're a central motif in stories from around the world and throughout time. In modern storytelling, when the monster shows up (whether Godzilla, zombies, a giant shark, or a serial killer), there's no question -- it's time to run. The monster is, itself, a carrier of evil intent, and means to do us harm. Today, monsters seem to lie toward the opposite end of the emotional spectrum from the disabled population, portrayed as helpless sufferers who deserve our pity and outpourings of charity.
But feelings of horror and feelings of pity are just two halves of a double-edged sword. Both cause us to recoil from the object of our attention. And both emotions mark who is a companion and who is an outsider. It's more comfortable, for many people, to donate money to a telethon in support of "the homebound," than it is to see a disabled person sitting at the table next to them in a restaurant.
And, according to The Online Etymology Dictionary, when "Monster" first entered written English (circa 1300) it originally meant a "malformed creature, afflicted with a birth defect." It came into English from Old French, and it came into the French from the Latin monstrum: an omen or portent, from monere: to warn (the same root that gives us "monitor" and "demonstrate").
To the ancient Roman priests and soothsayers, monsters were not, themselves, full of wrath or hatred, the way they are for us. Instead, malformed offspring were seen as a sign that the gods were angry, warning us to prepare for divine punishment.
Human psychology being what it is, however, such transference is predictable. Ostracizing the disabled, and denying our existence within a society, probably stems from a desire to deflect Divine Wrath:
"We're not the people who're sinning... no, nope. You warned us -- sent us an omen, Jove? You sure? 'Cause no such monster was born around here. You must be looking for that other village, over in the next valley."
If the creature insists on trying to return to the village, after being sent away, then clearly, it does harbor us ill-will, and wants Jove, or God, to strike us with lightning or pestilence. And thus, we come to the modern sense of monster, and all the stories where the hero must fight off the supernatural carrier of doom.
Just as often, though (and perhaps more often), the hero of the story is the "marked" child him or herself, shunned and outcast from the family, who must find their way back to the village, to overturn society, and begin a new era.
And sometimes, these "omens and portents," these "malformed creatures" come to us in the guise of the storytellers themselves.
"Monster" entry at the Online Etymology Dictionary, (c) 2001 - 2010, Douglas Harper: http://www.etymonline.com/index.php?search=monster&searchmode=none