The killers in Scream 2 are Mickey, a film student played by Timothy Olyphant, and the reporter Debbie Salt, played by Laurie Metcalf, who turns out to be Nancy Loomis, the mother of Billy from the first film. Everything below is a full spoiler.
After the Stab screening murders, Cici (Sarah Michelle Gellar) is killed at a sorority house. Sidney is later attacked, and her boyfriend Derek (Jerry O'Connell) is hurt. Gale notices that the victims' names echo those of the original Woodsboro victims, which confirms the copycat pattern. Ghostface then kills Randy Meeks (Jamie Kennedy) inside Joel's van, and Joel leaves town. Dewey is stabbed while reviewing Joel's footage with Gale.
Ghostface goes on to kill the officers escorting Sidney and Hallie (Elise Neal), and Hallie dies as well. Sidney finds Derek tied up in the campus theater, and Ghostface is unmasked as Mickey, who shoots Derek. Mickey wants to be caught and to blame movie violence so that he becomes famous. His partner Debbie Salt turns on him and shoots him, then reveals herself as Nancy Loomis. She wants revenge on Sidney, and she says she killed Randy for slandering her son.
Cotton Weary (Liev Schreiber) arrives and shoots her in return for an interview. Mickey comes back, and Gale and Sidney kill him. Sidney then shoots Nancy, and the threat ends.