Silent Night, Deadly Night

Little Billy witness his parents getting killed by Santa after being warned by his senile grandpa that Santa punishes those who are naughty. Now Billy is 18, and out of the orphanage, and he has just become Santa, himself.
Billy Chapman, who was traumatized by his parents' Christmas Eve murder, then brutalized by sadistic orphanage nuns, grows up to dress as jolly St. Nick for a yuletide rampage to punish the naughty.