The Killer Santa is the minor villain in Silent Night, Deadly Night and he was the responsible for killing Billy's parents in 1971 at Christmas Eve.
While Billy's family went to see Grandpa Chapman, a man disguised as Santa Claus robs a liquor store and shoots the clerk after he reaches for his gun.
Billy and his family were heading home until they saw the man with his car apparently broken down on the dark, deserted road.
Against Billy's terrified objections, his father pulls up beside the stranded man, who pulls out his gun and shoots Billy's father. Billy runs into a field, hides and watches as the criminal drags Billy's mother from the car and slits her throat, killing her.
He was one of the people that made Billy snap and he said to Billy "Where are you, you little bastard" and he continued his killing spree until the police finally managed to arrest him and send him to prison for life for his crimes.