Clowns as evil rather than playful have become an established character in popular culture. Most notable icons of evil clowns include John Wayne Gacy, a serial killer who killed 33 young men and often appeared at parties as Pogo the Clown and Pennywise the manifestation of the evil monster in Stephen Kings novel (and subsequent movie) It. There are also musical groups such as The Insane Clown Posse.
There are two major reasons clowns have become entrenched as signifiers of horror. One is that a common device in tales of horror is to find make the mundane and ordinary creepy. The more innocent the thing or character usually is the more shocking its transmutation to evil. (see the entry on Toys).
The other reason for clowns being objects of horror is that many of them are actually evil e.g. John Wayne Gacy. There is also deep suspicion these days around any man who works in a profession that focuses on children, and some clowns have been involved in child abuse.
The evil clown can also be seen as a signifier of a loss of innocence, a sign of the end of childhood. In 2.02 Everybody Loves a Clown Sam and Dean are grieving for their father, trying to cope with the loss of their only parent. It is a time of transition for them; it marks their passage into adulthood.
- see also Coulrophobia