He's making a list, he's checking it twice, and if you've been naughty your Christmas won't be so nice. According to ancient German folklore, while St. Nicholas was visiting the homes of good little girls and boys to reward them with toys and sweets, Krampus visited the bad children to mete out their punishment.
Sometimes Krampus would leave coal and switches in a naughty child's boots for their parents to swat them with. Those were the kids that got off easy.
Krampus carried switches to punish you with himself if you were bad enough. Imagine waking to a hoofed and horned demon with red eyes and a forked tongue, pulling you out of bed to lash your hide.
Worse yet, sometimes Krampus would bring a wash tub to drown you in, or a sack to carry you off to a cave where he would eat you. The very worst boys and girls would be dragged out of their beds and carried to Hell.
Christmastime had something for everyone to look forward to.