The modern archetype of the evil clown was popularized by DC Comics character the Joker starting in 1940 and again by Pennywise in Stephen King's It. The character can be seen as playing on the sense of unease felt by sufferers of coulrophobia, the fear of clowns.
See also: Sad clown paradox, Bad Ben, John Wayne Gacy, Crazy Clown Time, Shakespearean fool, and It (miniseries).