Deadly Friend

Teenage science genius Paul Conway (Matthew Laborteaux) and his single mother Jeannie (Anne Twomey) move into their new house in the town of Welling. He soon becomes friends with newspaper delivery boy Tom Toomey (Michael Sharrett). Living next door to Paul is the beautiful Samantha Pringle (Kristy Swanson) and her abusive, alcoholic father Harry (Richard Marcus). Paul built a robot named BB (Charles Fleischer), which occasionally displays autonomous behavior, such as being protective of Paul. Paul, Jeannie, and BB meet Paul's professor, Dr. Johanson (Russ Marin), at Polytech, a prestigious university Paul has a scholarship at. Dr. Johanson gives them a tour of the new laboratory Paul will have access to.
While Tom helps Paul teach BB how to deliver newspapers, they stop at the house of reclusive harridan Elvira Parker (Anne Ramsey), who threatens the boys with a double-barreled shotgun and expresses instant dislike for BB. Walking away, the three encounter a motorcycle gang led by bully Carl (Andrew Roperto). When Carl intimidates Paul by pushing him into garbage, BB assaults him by grabbing his crotch. The gang rides away with Carl vowing revenge.
Paul, Samantha, Tom, and BB begin to develop a close friendship, much to Harry's annoyance. One day, while playing basketball, BB accidentally tosses the ball onto Elvira's porch. She stomps out of her house and takes the ball, refusing to give it back, with BB taking note of Elvira's hostile attitude. On Halloween night, Samantha comes over with a bloody nose (presumably from her father's abuse) and asks for ice. Samantha goes out with Paul, Tom, and BB. Tom decides to pull a prank on Elvira. BB unlocks her gate and Samantha rings her doorbell. When alarms go off, they hide in a shrubbery nearby. When Elvira sees BB standing near her porch, she shoots him with her shotgun without hesitation, destroying him. Paul is devastated by the loss of his robotic friend.
On Thanksgiving Day, Samantha joins Paul and Jeannie for dinner. Afterwards, Paul and Samantha share their first kiss. Samantha returns home late at night, outraging her father. He punches her and pushes her down the stairs. At Polytech's hospital, Paul learns that Samantha is now brain-dead and will also be on life support for 24 hours, after which the plug will be pulled. As BB's microchip can interface with the human brain, Paul decides to use it to revive Samantha. The boys enter the hospital using the key taken from Tom's father, who works as a security guard there. After Tom deactivates the power from the basement, Paul takes Samantha to his lab. He inserts the microchip into Samantha's brain and takes her back to his house, hiding her in the shed. After he activates the microchip, Samantha "wakes up", but her mannerisms are completely mechanical, suggesting BB is in control of her body.
The police arrive at Samantha's home and inform Harry that her body has disappeared. In the middle of the night, Paul finds Samantha staring at the window, looking at her father, and he deactivates the microchip. The next morning, Paul awakens and finds Samantha gone. He searches for her in the street to no avail. Samantha goes back to her house and down the cellar. When Harry finds the cellar door open and goes downstairs, she attacks him, breaks his wrist and snaps his neck. Paul finds Samantha and Harry's corpse in the cellar. Horrified, he hides the body, takes Samantha back to his home and locks her in his bedroom. At night, Samantha breaks into Elvira's house and attacks her by throwing her at the wall of her living room. As a terrified Elvira screams for her life, Samantha kills her by throwing the basketball she stole from Tom at her head, causing it to explode. Elvira's headless corpse then walks around the living room until it collapses.
When Tom learns of Samantha's rampage, he and Paul get into a fight. Tom threatens to call the police and end this once and for all. Still being protective of Paul, Samantha jumps out the attic window and attacks Tom, with Paul and Jeannie intervening. Trying to get her under control, Paul slaps Samantha, which results in her trying to strangle him. Samantha then lets him go and runs away. As Paul goes after her, he again encounters Carl, who gets into a fight with him. Samantha goes back for Paul, grabs Carl and throws him at an incoming police car, the impact killing him. She runs back to Paul's shed, where Paul comforts her and realizes she's regaining some of her humanity. However, the police arrive with their weapons pointed at Samantha, who yells out Paul's name in her human voice. She runs towards him, trying to protect him, with Sergeant Volchek (Lee Paul) thinking she's trying to attack him and shooting her. She says Paul's name one more time before dying in his arms.
Later at the morgue, Paul tries to steal Samantha's body once more, not having learned his lesson. Suddenly, Samantha grabs Paul's neck and her face rips apart, revealing a terrifying variant of BB's head. Her skin strips away, revealing half-robotic bones underneath. With a robotic voice, Samantha tells him to come with her. When Paul refuses, she snaps his neck off-screen, killing him.

Question: Who pushes Samantha down the stairs?
Answer: Her father.