Silent Night, Deadly Night

In 1971, 5-year-old Billy Chapman and his family visit a nursing home in Utah where his catatonic grandfather lives. When Billy's parents leave the room, his grandfather suddenly awakens and tells Billy to fear Santa Claus, as he punishes the naughty.
On the way back home, a criminal dressed in a Santa suit – who had just robbed a liquor store and killed the owner – attempts to carjack the family. As Billy's father tries to drive away, the criminal shoots him dead and attempts to sexually assault Billy's mother; when she hits him, he slits her throat with a switchblade. Billy flees and hides, leaving his baby brother Ricky in the car.
Three years later, in December 1974, 8-year-old Billy and 4-year-old Ricky are celebrating Christmas in an orphanage run by Mother Superior, a strict disciplinarian who beats children who misbehave and considers punishment to be a "good" thing. Sister Margaret, who sympathizes with the children, tries to help Billy, but he is regularly punished. On Christmas, the orphanage invites a man in a Santa Claus suit to visit the children; Billy, forced to sit on his lap by Mother Superior, punches the man before fleeing to his room in horror.
10 years later, in the spring of 1984, Billy, now 18, leaves the orphanage for a normal life, and obtains a job as a stock boy at a local toy store, with support from Sister Margaret. At the store, he develops a crush on his co-worker Pamela; he has sexual thoughts which are often interrupted by morbid visions of his parents' murders. On Christmas Eve, the employee who plays the store's Santa Claus is injured and Billy's boss Mr. Sims makes him take his place. After the store closes, the staff has a Christmas Eve party. Billy, still in a Santa Claus suit, tries to have a good time, but keeps having memories of his parents' murders, causing him to feel depressed. He sees his co-workers Andy and Pamela kissing and then walking into the back room. Billy follows them and sees Andy trying to rape Pamela. This triggers his insanity; he hangs Andy with a string of Christmas lights and, declaring that punishment is "good", murders Pamela with a box cutter.
Next, Billy murders Mr. Sims and his manager Mrs. Randall. Sister Margaret discovers the carnage and goes to the police station to seek help. Billy breaks into a nearby house where a young couple named Denise and Tommy are having sex and a little girl named Cindy is sleeping; he impales Denise on a set of deer antlers and throws Tommy through a window. When this awakens Cindy, Billy asks if she has been nice or naughty; she says nice, and he gives her the box cutter he had used earlier. After this, he witnesses bullies picking on two teenage boys and stealing their sleds, and decapitates one of the bullies with his axe.
The next morning, Captain Richards and Sister Margaret deduce Billy will go to the orphanage, where Ricky is still living. Officer Barnes responds to orders to secure the orphanage and kills a pastor, Father O'Brien, who was dressed in a Santa outfit, mistaking him for Billy. As Barnes continues patrolling the area, he is struck in the chest by Billy's axe. Billy confronts Mother Superior, now in a wheelchair. Just as he prepares to kill her, Richards shoots him in the back. Billy drops to the floor and tells the children "You're safe now, Santa Claus is gone", before dying from his injuries. A 14-year-old Ricky, scowling at Mother Superior, utters "Naughty."