I Spit On Your Grave

Manhattan short story writer Jennifer Hills (Camille Keaton) rents an isolated cottage in Kent, Connecticut on Park Hill Lane near the Housatonic River in the Litchfield County countryside to write her first novel. The arrival in town of the attractive and independent young woman attracts the attention of Johnny (Eron Taber), the gas station manager, and Stanley (Anthony Nichols) and Andy (Gunter Kleemann), two unemployed men who hang around the gas station. Jennifer has her groceries delivered by Matthew (Richard Pace), who is mildly mentally disabled. Jennifer is wearing tight clothing and Matthew peeps at her breasts. Matthew is friends with the other three men and reports back to them about the beautiful woman he met, claiming he saw her breasts.
Stanley and Andy start cruising by the cottage in their boat and prowl around the house at night. One day, while Jennifer is relaxing in her canoe, they surprise her in their speedboat and tow her to shore. As she tries to escape, she is met by Johnny, while Matthew hides in bushes nearby. She realizes they planned her abduction so Matthew can lose his virginity. Jennifer fights back but is chased by the men through the forest, they then rip her bikini off and hold her to the ground. But Matthew refuses to have sex with her, so Johnny rapes her. They allow her to escape but track her down shortly afterward. Andy proceeds to anally rape her; after she crawls back to her house, they attack her again. Matthew finally rapes her after drinking alcohol, but he says that he can not reach orgasm with the other men watching him. The other men ridicule her book and rip up the manuscript, and Stanley sexually assaults her. She passes out; but, after the men leave, Johnny realizes she is a witness to their crimes and orders Matthew to stab her to death. Matthew cannot bring himself to do this, so he dabs the knife in the blood on her cheek and returns to the other men, claiming he has killed her.
In the following days, a traumatized Jennifer pieces both herself and her manuscript back together. She goes to church and asks for forgiveness for what she plans to do. The men learn Jennifer has survived and beat Matthew up for deceiving them. Jennifer calls in a grocery order, knowing Matthew will deliver it. He takes the groceries and a knife. At the cabin, Jennifer entices him to have sex with her under a tree. As he becomes oblivious to his surroundings, she strings a noose around his neck and hangs him, then cuts the rope and his body drops into the shallow edge of the lake.
At the gas station, Jennifer seductively directs Johnny to enter her car. She stops halfway to her house, points a handgun at him, and orders him to remove all his clothing. Johnny insists the rapes were her fault because she enticed the men by parading around in revealing clothing. She pretends to believe this as Johnny grabs the gun from her hand and throws it to the ground. She invites him back to her cottage for a hot bath, where she gives him a handjob. When Johnny says that Matthew has been reported missing, Jennifer states that she killed Matthew; and, as he nears orgasm, Jennifer takes the knife Matthew brought with him from its hiding place under the bathmat and severs Johnny's genitals. She then leaves the bathroom, locks the door behind her, and listens to classical music as Johnny screams and bleeds to death. She later dumps the body in the basement and burns his clothes in the fireplace.
Stanley and Andy learn that Johnny is missing and take their boat to the shore of Jennifer's cabin. Andy goes ashore with an axe. Jennifer swims out to the boat and climbs aboard before Stanley realizes she is there and pushes him overboard. Andy tries to attack her when she speeds past him in the boat, but she escapes with the axe. Andy swims out to rescue Stanley, but Jennifer plunges the axe into Andy's back, killing him. While the outboard motor is off, Stanley moves towards the boat and grabs hold of the motor to climb aboard, begging Jennifer not to kill him. She repeats the same words that he used against her during the sexual assaults: "Suck it, bitch!" Jennifer then starts the motor and traps Stanley in the spinning propeller, disemboweling him. Jennifer then speeds away in the boat as the film ends.

Question: Why was Matthew reported missing?
Answer: Jennifer killed Matthew.