Childhood as an ideal represents nothing so much as innocence in its purest form. And innocence itself is the ultimate distillation of “good”. Perhaps this is why both creators and audiences alike have often had something of a difficult time dealing with it within the horror medium. Because childhood represents the ultimate good, the corruption/destruction of that good is the most extreme form of evil that most of us can imagine. Very often it is simply too much to bear.
This is why, for as long as horror films have been around, the ultimate taboo, the one area most have avoided like the plague, has been the murder of children. True, there have been notable exceptions over the years, movies like Frankenstein (1931), The Blob (1988), and Sleepy Hollow (1999). But for the most part, filmmakers keep away from it, as exemplified most vividly in some of the Friday the 13th movies, in which Jason will literally walk past the beds of sleeping campers and keep his focus on the counselors. For most of us, violence against children is something we don’t really want to see in horror movies. It’s not fun or entertaining, and unfortunately, it's all too painful and real.
Which brings me to the original topic: evil kids in the horror genre. Ruling out the literal destruction of the child, the closest most horror creators choose to come is the destruction of childhood. If horror is all about the corruption of good, then the corruption of the ultimate good, the innocence of childhood, is about as evil as it gets.
For this reason, the depiction of evil children stirs up deep feelings of dread and revulsion in many viewers. We innately perceive it as a gross affront to the natural order of things. Something within us senses this perversion, and recoils from it. Evil adults we can handle; most of us deal with them on an almost daily basis. But evil children? And by this I don’t mean the bratty kid on line at the grocery store who won’t shut up — I mean genuinely, truly evil children. An utterly alien concept.
Some of the genre’s finest works have mined this motherlode of subconscious terror: The Omen (1976), Halloween (1978), The Ring (2001), and most recently, The Orphanage (2007). It works to particular effect in William Friedkin’s masterpiece The Exorcist (1973), in which we literally witness the purest and most innocent little girl imaginable defiled and twisted by a wholly evil force into an obscene mockery of nature. Though flawed, Stephen King’s Pet Sematary (1989) pulls off a powerful combination by presenting us with the ultimate taboo (death of a child), followed by the perversion of innocence, as the child returns in evil form.
In short, it is this underlying sense of profound and incomprehensible wrongness that causes us to fear the so-called “evil child” in horror movies. It is also the subconscious connection to the ultimate act of corruption — the literal corruption of the flesh itself, i.e. the death of the child. Sublimating this primordial horror in the form of corrupted childhood thus becomes a safer way to scare the crap out of us, without offending.