Stupidity: Ghostface ambushes Sidney and her friend with the two detectives protecting her while in the car. Ghostface busts out of the driver window, and slashes the throat of the driver detective and then assaults his partner, kicking the crap out of him. Ghostface is obviously going with the intent to kill and has a weapon. He just killed one of the detectives and threw the other in the street in front of the car. When Ghostface jumps in the car to take off, the living detective, beaten, jumps up with his gun trained on Ghostface. But rather than do anything smart, he just yells at the killer, the cop killer, to freeze and get out of the running car that he's standing in front of. He just saw the guy kill his own partner and knows he is intending to kill the girls in the back. His gun is pointed at Ghostface who is about to drive over him, but he just yells at him instead of shooting.
Stupidity: Gale and Dewey review Joel's footage. Joel is technically a professional cameraman working for a local network affiliate. Inexperienced, but he's still working with a professional camera, he has his own van...and yet the footage the two are watching is of poor quality; it would have been unusable for the TV station. The killer's footage, shot with inferior equipment and under more trying circumstances, is the same quality if not better. (01:17:00)
Stupidity: Sidney and Hallie run away as soon as they escape the cop car, when Sidney could've easily grabbed the cop's gun off the hood and shot the supposedly incapacitated killer, or at least held them at gunpoint and removed their mask. (01:30:50)
Stupidity: When the Jada Pinkett and her boyfriend are outside the theater, they are part of a long line. They get their tickets at 9:50, but when they enter the movie, which is in a one-night only sneak preview event, is already running. The event in general is completely absurd especially for a premiere, since everyone in the audience acts like savages (but they shut up Maureen the moment she says something). It is also a movie based on a gruesome tragedy with real people killed barely a couple years before, where they hand stabby props.
Answer: His main motive was fame, or infamy for that matter.