Audio problem: When Dewey is being stabbed up against the soundproof glass, you see a shot of Gale shouting "No." You can see a reflection of the killer in the glass standing motionless, but you can hear Dewey being stabbed. (01:18:45)
Audio problem: At the theater showdown, Mickey shoots Gale and Sidney says, "Gale," but her mouth is closed.
Audio problem: When Gail and Dewy are watching the tape of their conversation in the park, the sound of their conversation is way too clear taking into account their distance from the camera and the microphone, and there is no ambient noise at all. (01:14:15)
Audio problem: The killer replays for Gale and Dewey Randy's death. The words he uses are the same, but the pacing of his last line is off, and so is the part before; in the original scene Ghostface finished the "You'll never get the girl" line just as Randy touched the phoning bystander, but in this footage the line ends earlier than that moment. (01:18:45)
Audio problem: Randy is leaving class to meet Sidney, but his classmate first asks him "So, Mr. Originality..." The camera cuts away before she finishes the sentence, but you can tell that the word that came after "So" doesn't match what the actress is saying. (00:17:20)
Answer: His main motive was fame, or infamy for that matter.