Halloween

On Halloween night of 1963, six-year-old Michael Myers, dressed in a clown costume and mask, stabs his teenage sister Judith to death with a kitchen knife. Fifteen years later, on October 30, 1978, he escapes from Smith's Grove Sanitarium in a car that was to take him to a court hearing, the verdict of which was for him to be locked up forever.
The next day, 21-year-old Michael (Nick Castle), now wearing a mechanic's uniform and a white mask, returns to his hometown of Haddonfield, Illinois and begins stalking high school student Laurie Strode (Jamie Lee Curtis) after she drops off a key at his old house (where the murder took place) so her father can sell it. Her two best friends Annie Brackett (Nancy Loomis) and Lynda van der Klok (P. J. Soles) dismiss Laurie's concerns that someone is following her. Michael's psychiatrist, Dr. Sam Loomis (Donald Pleasence), having anticipated Michael's return home, goes to the local cemetery and finds Judith Myers' headstone missing. Loomis meets with Annie's father, Sheriff Leigh Brackett (Charles Cyphers) and monitors the local neighborhood to keep an eye out for Michael.
When Annie's boyfriend Paul calls her to come pick him up, she takes Lindsey Wallace (Kyle Richards) to the Doyle house, just across the street, to spend the night with Laurie and Tommy Doyle (Brian Andrews). Michael strangles Annie after she gets into her car. While playing hide-and-seek with Lindsey, Tommy sees Michael carrying Annie's body into the Wallace house, and, believing he's the "boogeyman", tries to tell a skeptical Laurie about what he saw. Later, Lynda and her boyfriend Bob enter the Wallace house to spend the night together. After having sex, Bob goes downstairs to get a beer for Lynda, but Michael ambushes and impales him. He then poses as Bob in a ghost costume and confronts Lynda, who kids around with him. After no response, she calls Laurie; just as Laurie answers the phone, Michael strangles Lynda with the telephone cord. Meanwhile, Dr. Loomis discovers the car Michael had stolen, confirming his beliefs that his former patient is in the neighborhood.
Getting suspicious, Laurie goes over to the Wallace house to see what her friends are up to and is horrified after finding them dead in the upstairs bedroom. Michael suddenly appears and slashes her arm, causing her to fall over the railing. She recovers and barely escapes. Running back to the Doyle house in a panic, she finds that the keys are gone and the door is locked. When Laurie sees Michael coming, she throws a potted plant up at the window to wake Tommy up and desperately asks him to open the door quickly. He opens the door just in time and lets her in and she tells him to hide upstairs with Lindsey. Michael, who got in through an open window, attacks her, but she stabs him in the neck with a knitting needle and he collapses.
Laurie goes upstairs and tells Tommy and Lindsey she killed the "boogeyman"; Tommy says "You can't kill the boogeyman" as Michael appears behind her. Tommy and Lindsey lock themselves in the bathroom while Laurie hides in a bedroom closet. Michael destroys the closet door to get to her, but she undoes a metal clothes hanger and sticks him in the eye with it. He drops the knife and she stabs him with it and he collapses again. Laurie exits the closet and asks the kids to go to the neighbor's house and have them call the police. Michael gets up again. Dr. Loomis sees Tommy and Lindsey flee the house, screaming in terror, and enters the house to find Michael tussling with Laurie upstairs. Laurie pulls off Michael's mask and he stops attacking her to put it back on. Loomis shoots Michael six times and he falls over the second story balcony ledge. Laurie, in shock, concludes "It was the boogeyman." and Loomis replies: "As a matter of fact, it was." Loomis looks down over the balcony to see that Michael is gone. Places where Michael had previously been are shown as his breathing is heard, indicating he could be anywhere.

Question: Who is the "boogeyman"?
Answer: Michael