One comes to a marathon of the “Terrifier” films much as you might approach a rickety rollercoaster at a fading state fair, skeptical, but secretly hoping to stumble out a little changed. It all begins with a shriek and a snicker: Damien Leone’s creation, Art the Clown, born of a 2011 short that was less a calling card than a threat. Yet by the third film, set amid Christmas glitter and moral queasiness, the series has grown into a twisted, almost baroque pageant of splatter and dark wit. The “Terrifier” films, watched in quick succession, form their own fevered treatise on what horror can still provoke, where it dare not go, and just how long you can keep laughing in the dark before you start to worry about yourself.