Scream 4

On the fifteenth anniversary of the original Woodsboro murders, high school students Jenny Randall and Marnie Cooper are attacked and brutally killed by a new Ghostface while watching Stab 7, a "film within a film" which itself has a "film within a film".
The following day, Sidney Prescott returns to Woodsboro to promote her new book with her publicist Rebecca Walters. After evidence is found in Sidney's rental car, Sidney becomes a suspect in the murders and must stay in town until the murders are solved. Sidney's teenage cousin Jill, who is dealing with the betrayal of her ex-boyfriend, Trevor Sheldon, gets a threatening phone call from Ghostface. She and her friends Olivia Morris and Kirby Reed, are questioned about their calls by Dewey Riley, who is now the sheriff of the town, while one of his deputies, Judy Hicks, assists him in the case. Meanwhile, Dewey's wife, Gale Weathers-Riley, is struggling with writer's block. She gives up the writing and decides to investigate the murder instead.
Sidney stays with her aunt Kate Roberts. Later that night, Olivia, who lives next door to Jill and Kate, is attacked and brutally murdered by Ghostface as Jill and Kirby watch in horror. Sidney and Jill rush in to save Olivia, but the killer injures them and gets away. Sidney and Jill are rushed to the hospital. In the hospital's parking garage, Ghostface murders Rebecca. Gale, trying to solve the murders, enlists the help of two high school movie fanatics Charlie Walker and Robbie Mercer, who explain that the killer is using the rules of movie remakes to murder. Charlie concludes that the killer will likely strike at a party being held that night.
Gale goes to the party to investigate to which Ghostface attacks and stabs her but flees when Dewey arrives. Dewey takes her to the hospital. At Jill's house, Sidney discovers that the policemen assigned to guard the house are dead. She also discovers that Jill has left the house and gone to Kirby's. Sidney goes down to tell Kate, but the killer appears and chases her and Kate. In the ensuing attempt to keep Ghostface from entering the house, Kate is stabbed and killed. After Deputy Judy Hicks arrives, Sidney rushes to Kirby's house to save Jill on her own.
Jill, Kirby, Charlie, Robbie and Trevor are at Kirby's house when Ghostface appears and stabs and kills a drunken Robbie. Sidney arrives at the house. Kirby is forced to answer horror movie trivia to save Charlie, who is tied up outside. Sidney goes upstairs to find Jill, promising to return to Kirby. After Kirby answers Ghostface's questions, she goes outside to untie Charlie, believing that she has won the game. He suddenly stabs her in the stomach and reveals himself as Ghostface, before leaving her for dead. Sidney is then confronted and subdued by Charlie, but Sidney manage to escape and is then stabbed in the thigh by a second Ghostface, who is revealed as Jill. They explain that they committed the murders as a part of a remake of the first murders. Jill further reveals that she did it out of anger and jealousy of Sidney's fame and she wants the attention Sidney got for surviving the murders. They also reveal that they intend to frame Trevor as Ghostface. Charlie then pulls Trevor out of a closet who is then shot by Jill in groin for cheating on her and then in the head. Charlie asks Jill to stab him non-fatally to make them look like victims; Jill responds by stabbing him to death, intending to pin him as Trevor's accomplice and to make herself the sole survivor. Jill stabs Sidney in the stomach and begins to harm herself to make it seem as if Trevor attacked her. Later, Dewey, Judy, and the rest of the police stumble upon the carnage.
Jill is then taken to the hospital. After discovering that Sidney had survived, she goes to Sidney's hospital room and attempts to finish her. Dewey, Gale, and Judy intervene, having been clued by a detail about Gale's injury that Jill somehow knew. Jill subdues all of Sidney's rescuers with Dewey's gun, which gives Sidney the chance to electrocute Jill and then shoot her in the chest, finally killing her. Dewey calls in all police units, as media reporters outside confirm Jill as the "sole-surviving hero" and a "hero, right out of the movies" giving Jill the fifteen minutes of fame she so desperately craved.

Question: Who kills Robbie?
Answer: Ghostface