House of Wax

In 1974, a woman makes a wax sculpture as one of her sons eats breakfast. Her husband enters with another son. After being strapped and taped into his chair, he scratches his mother's hand. Then she slaps the child across the face.
In 2005, Carly Jones, her brother Nick, her boyfriend Wade, her best friend Paige, Paige's boyfriend Blake, and Nick's friend Dalton are on their way to a football game in Louisiana. When night falls, the group sets up camp. A stranger in a pickup truck visits their campsite, shines his lights, and refuses to leave or address them until Nick smashes one of his headlights. The next morning, Wade discovers that his fan belt is broken. Carly and Paige wander into the woods, and Carly falls down a hill, into a pit filled with animal carcasses. After rescuing her, the group meets a strange, rural man named Lester, who offers to drive Carly and Wade to the nearby town of Ambrose to get a new fan belt, while the rest of them go to the football game.
The two arrive at Ambrose, which is virtually a ghost town. Unable to find an attendant at the gas station, they wander into the church, disrupting a funeral. There, they meet a mechanic named Bo, who offers to sell them a fan belt after the funeral. While waiting for the services to end, Carly and Wade visit the wax museum, which itself is made of wax and is the central feature of the town. The gas station does not have the right size fan belt, so they follow Bo to his house. Wade is knocked out by Bo's brother, Vincent. Outside, Carly realizes Bo is the one who visited them the night before, after noticing the broken headlight. She runs to the church for help, but finds that it is populated only by wax sculptures. Bo captures her and brings to the cellar of the gas station, where she is restrained. Meanwhile, Wade is stripped naked and strapped to a chair to be covered in wax.
Nick, Dalton, Paige, and Blake realize they will not arrive at the game in time and turn around. Nick and Dalton arrive in Ambrose to find Carly and Wade. Nick visits the gas station, where he questions Bo about Carly's whereabouts. When she tries to gain Nick's attention, Bo cuts off the tip of Carly's finger, but she tears her glued lips apart and screams for help. Nick fends off Bo and frees Carly. Meanwhile, Dalton finds Wade, who is still alive but unable to move due to the wax. Attempting to peel off the wax, Dalton realizes that he is unintentionally removing Wade's skin in the process. Vincent finds Dalton and slashes Wade's face with a machete, killing him. After a chase through the museum, Vincent catches Dalton and decapitates him. Meanwhile, Nick and Carly realize that all of the town's inhabitants are real people covered in wax; Bo and Vincent have been luring people in and covering them in wax to make the figures look more realistic.
At the campsite, Vincent kills Blake and chases Paige to a sugar mill, where he kills her. Nick and Carly return to the house to find Wade and Dalton. When Bo and Vincent return, they chase Carly and Nick to the House of Wax. It is revealed that Bo is the "evil" brother and Vincent is the "good" one; since their parents died, Bo has been controlling the more mild-mannered Vincent through physical and verbal abuse. After a chase scene, Carly beats Bo to death with a baseball bat. Nick unintentionally sets the House of Wax on fire, and the wax figures start to melt, as does the whole house. An enraged Vincent chases Carly to the top floor, but Carly and Nick stab Vincent. His body falls through the floor, and he lands on top of his brother's corpse. Carly and Nick escape the House of Wax as it melts to the ground.
The next day, the police arrive and report that Ambrose has been abandoned for ten years when the local sugar mill failed. As Nick and Carly are taken to a hospital, the policeman reveals Mr. and Mrs. Sinclair had a third son. From inside the ambulance, Carly sees Lester, who smiles and waves goodbye.

Question: Why does the mother slap her son?
Answer: He scratches her hand.