Continuity mistake: In her opening shot, CICI says that you can't blame violence on entertainment. Behind her, out of focus, there's a student with a beige shirt. You can see his left hand, but in the next shot he has his right arm crossed on top of it. The rest of the scene goes on with that extra's motions not matching at the cuts (he's touching his chin next time, then he isn't). (00:15:20)
Continuity mistake: When Sidney grabs the caller ID device, the position of her fingers on it change between shots. (00:12:30)
Continuity mistake: When Maureen's boyfriend reveals not to be Maureen's boyfriend at all, the height he holds his knife at changes between shots. (00:10:25)
Continuity mistake: When Maureen goes back to her seat and starts putting on her jacket, the dude at her side begins waving his arm super-enthusiastically making happy coos. She finishes putting her jacket on and the guy, who is still making those sounds in dub, is sitting composed. (00:07:05)
Continuity mistake: When Maureen and Phil arrive in the theater, immediately at the beginning before the camera even gets fully in focus you can see by the door a blonde girl putting on the Ghostface hood, with her partner helping her. The camera cuts away for a second, and in the next wide shot you see her repeating the gesture. (00:01:40)
Answer: His main motive was fame, or infamy for that matter.