Continuity mistake: When Sidney is about to get in the car with the policemen, she has a rather large noticeable spot on her chin. In the following scenes, it is gone.
Continuity mistake: When Gale kisses Dewey she has no more lipstick on her mouth, then in the next shot she has lipstick on.
Continuity mistake: A pimple is visible on Sidney's chin as she says goodbye to Derek before getting into her ride with the two detectives and Hallie. Minutes later, in the scenes inside the car, the pimple has completely disappeared, even in the very extreme closeups.
Continuity mistake: After the accident - when the car is seen from behind - you can't see the killer (it's before Sid crawls over him).
Continuity mistake: In the make-out scene with Gale and Dewey, Gale's lipstick is smeared on her face, but there's none on Dewey's face.
Answer: His main motive was fame, or infamy for that matter.