Yin must have Yang
Our universe is built upon the number two. In our cells, DNA is composed of two helical threads, matter has its opposite, antimatter. Computers run on a code built on two digits - one and zero.
Likewise, religions are built around good and evil, two forces that oppose each other in both action and purpose. God has his counterpart, Satan. Jesus has his opposite, the Anti-Christ. This pattern even shows up in polytheistic religions; Thor has his counterpart, Loki, and the Olympian gods in Greek mythology have their opposites, the Titans.
However, one of our most popular mythological figures, Santa Claus, has lost the tale of his opposite. Called Krampus, this being dealt out punishments to the wicked children of the world. For the particularly nasty children, the Krampus administered the harshest of punishments: abduction and death.
Over time, as mankind's command of its realm expanded, our technologies and our own actions took the places that were once held by mythological beings. Nowadays, parents indulge their children with gifts, displacing the old Santa Claus.
However, there exist parents who snap and do unthinkable things to their children. Every year, our news media broadcasts stories of parents who went over the edge. Almost like they were possessed by a demon...
With no Santa Claus to reward the righteous, and no punishment for the wicked, both forces are unleashed indiscriminately against good and evil alike. For every naughty child rewarded, a counterbalance must exist.
Yin must have a Yang.