In 1974, 11-year-olds Wendy Richards, Jude Cunningham, Kelly Lynch, and Nick McBride play hide and seek in an abandoned convent. 10-year-old Robin Hammond tries to join them, but they start teasing her by repeating "Kill! Kill! Kill!". Robin is then backed up towards a window from which she falls to her death. Instead of reporting the incident to the police, the children make a pact not to tell anyone what happened and keep the incident a secret, but after they leave, a shadow falls across Robin's body. Later a known rapist, Leonard Merch is mistakenly blamed for Robin's death and is arrested.
Six years pass, Robin's family attend her memorial on the anniversary of her death. Robin's teenage sister and brother, Kim and Alex, are also preparing for the school prom to be held that evening. Their parents will also attend, as their father is the school principal. Kelly, Jude and Wendy begin receiving obscene phone calls from an unknown figure, while Nick ignores his ringing phone. Kim and Nick, whom she is dating, are attending prom together; Jude is asked by goofy jokester Seymour "Slick" Crane whom she meets by chance that morning; Kelly is going with her boyfriend Drew Shinnick (who is preoccupied with having sex with her despite objections), while Wendy, previously Nick's girlfriend, asks the school rebel, Lou Farmer with plans to embarrass Nick and Kim at prom.
In the changing room after gym class, Kim and Kelly discover the locker room mirror severely cracked and a shard missing. The offender blamed for Robin's death has escaped and Lt. McBride, Nick's father, investigates. Also during the day, Wendy, Jude and Kelly discover their year book photos in their lockers torn and stabbed with a piece of glass. During the senior prom, Kim and Nick perform a dance number to impress Wendy who had insisted Nick would be getting back with her after the prom. Later, Kelly and Drew make out in the changing room, but Kelly refuses to continue to full sex, resulting in Drew angrily leaving. As Kelly gets dressed, an unidentified figure wearing a ski mask and all-black clothing stealthily approaches her and slits her throat with a mirror shard. Jude and Slick have sex and smoke marijuana in Slick's van parked outside school grounds. Unbeknownst to them, they are being watched and are then attacked by the masked killer, who stabs Jude's throat. Slick struggles with the killer while attempting to drive away but the killer escapes from the van (with Slick inside) as it tumbles off a cliff and explodes into a wreckage. McBride, staking out the prom, is informed that the sex offender blamed for Robin's death has been caught. He is relieved and ends his scrutiny of the event.
Wendy in the bathroom is then confronted by the killer now wielding an axe and is chased through the school. After evading the killer several times, she is suddenly caught and killed after she screams upon discovering Kelly's body in a storage room. Kim and Nick prepare to be crowned prom king and queen but Wendy's plan is put into action by Lou and his lackeys who tie up Nick with Lou taking his crown and assuming his position back stage. Thinking he is Nick, the killer approaches behind Lou and decapitates him. Lou's head lands onto the dance floor, sending the partygoers fleeing in horror.
Kim finds Nick and frees him. As they prepare to escape, they are confronted by the killer who attacks Nick but not Kim. Eventually in the ensuing brawl, Kim bluntly strikes the killer's head with his own axe. She and the killer then stare at each other for a moment and Kim realizes who he really is. He runs outside where the police have arrived. As guns are raised Kim screams for Lt. McBride not to shoot him. The killer then collapses on the ground and is revealed to be Alex, who is Robin's twin. He tearfully tells his sister that Jude, Kelly, Wendy and Nick were responsible for their sister's death. The film ends as he cries Robin's name and dies. Kim sheds a tear for the death of another sibling.

Question: Why was Alex murdering the other kids? 
Answer: Because they were responsible for Robin's death and he wanted revenge
Alternative answer: Because they caused his sister Robin's death years earlier.