Friday the 13th: The Final Chapter

The night after the events of Higgin's Haven, paramedics clean up the remains left by mass murderer Jason Voorhees and deliver him to the morgue. After reviving from his head wound and escaping from the cold storage, Jason kills a doctor by slicing his throat with a hacksaw, then guts a nurse with a scalpel.
The next day, a group of teenagers drive to Crystal Lake for the weekend. The group consists of Paul, his girlfriend Sam, Sarah, her boyfriend Doug, socially awkward Jimmy, and prankster Ted. On the way, the group comes across Pamela Voorhees's tombstone and a female hitchhiker. The hitchhiker is killed shortly after by Jason, who has returned to the area. The teens arrive and meet neighbors Trish Jarvis, her brother Tommy, and their dog Gordon. While going for a walk the next day, they meet two local girls, twin sisters Tina and Terri, and they go skinny dipping with them. Trish and Tommy happen upon the scene, and, before they leave, Trish is invited to a party to take place that night. Afterwards, their car breaks down, and Trish and Tommy meet a young man named Rob. They take him to their house, where Tommy shows Rob several monster masks he made himself before Rob leaves to go camping.
Later that night, the teens throw a party. Tina makes a move on Paul, which makes Sam jealous. Sam leaves for a swim but is stabbed through a rubber raft by Jason. Feeling guilty, Paul goes after Sam and discovers her body just before he is impaled in the groin with a harpoon gun. Terri rejects Teddy's advances and wants to leave, but Tina moves onto Jimmy, and the two go upstairs to have sex. Frustrated, Terri leaves the party, but she is brutally impaled outside by Jason.
At the Jarvis house, Mrs. Jarvis returns home to find the power out; when she goes out to investigate, she is startled by something she sees. While Sarah and Doug are having sex in the shower, Teddy finds an old stag film and brings it up on the projector. After sleeping with Tina, Jimmy comes downstairs and gets a glass of wine to celebrate. Jason appears, pins Jimmy's hand with a corkscrew, and slams a meat cleaver into his face. Upstairs, Tina notices Terri's bike is still there before she is grabbed through the window by Jason and thrown on top of the teens's car, killing her. A stoned Ted is then stabbed in the head with a kitchen knife through the projector screen. Jason moves upstairs, crushing Doug's skull in the shower. When Sarah returns and finds him dead, she tries to flee the house, but she is killed with a double-bit axe.
Trish and Tommy return from town and discover the power outage. Trish looks for their mother and orders Tommy to fix the lights. Trish comes across Rob's campsite and becomes worried when he says Jason is still alive; they return to the house. Running next door, they discover the bodies of the teens. Rob is caught and killed by Jason as Trish runs home. They barricade the house, but Jason breaks in and chases them into Tommy's room. Trish lures Jason out of the house while Tommy escapes. She runs through the teens' house, then out the upstairs window when he corners her. Returning to her own home, she is devastated to learn that Tommy is still there. She senses Jason behind her and tries to kill him with a machete, but she misses; he attacks her. Tommy, having disguised himself to look like Jason as a child, distracts him long enough for him to slam the machete into the side of Jason's skull, and he collapses to the floor. When Tommy notices that Jason's fingers are moving, he takes the machete and finally kills Jason by repeatedly hacking at his body, screaming, "Die! Die!" while Trish screams his name.
At the hospital, Trish insists on seeing her brother, who was traumatized by the events. He rushes in, hugs her, and gives a disturbed look while staring ahead.

Question: Who is the mass murderer?
Answer: Jason Voochees