Here's why Doofy became the original Ghostface killer in Scary Movie. The horror genre was in a bad place during the early 1990s, with the glut of slasher and monster movies during the 1980s leaving audiences slightly jaded with horror. The surprise success of 1996's Scream helped reinvigorate the genre and led to a string of similiar slashers with a dose of meta-humor. Other highlights from this period include I Know What You Did Last Summer and Urban Legend.
Scream's original title was Scary Movie, which was eventually used for its 2000 parody. The movie borrows a lot from Scream and its first sequel, with Anna Faris playing Cindy (Anna Faris), a teenager who - along with her friends - is stalked by a killer in a Ghostface mask. The movie not only parodies Scream though, as it mixes in everything from I Know What You Did Last Summer to The Matrix and Amistad.
Comedian Dave Sheridan played Officer Doofy in Scary Movie, a major character who acts as a riff on David Arquette's Dewey from the Scream franchise. In keeping with the movie's intentional bad taste tone, he's portrayed throughout as mentally challenged and often greets other characters with his Doofy salute. Scary Movie, like Scream, functions as a murder mystery, and the finale seems to reveal Cindy's boyfriend Bobby and their friend Ray were the killers. It's only at the end, when Cindy thinks it over with the sheriff and - in a riff on The Usual Suspects - realizes it was Doofy who was the original killer the whole time.
Dewey was faking his disability and murdered Carmen Electra's opening victim Drew - yet another nod to Scream, as Drew Barrymore played that film's first kill - for turning down his advances. This means there are three Ghostface killers in Scary Movie, with Doofy committing the opening murder, while Bobby and Ray kill the bulk of the other victims. Doofy then dons the costume again in the finale to kill Bobby and Ray and tries to kill Cindy because he had a crush on her too.
Just like The Usual Suspects' Verbal, Scary Movie's Doofy manages to get away with his crimes and didn't return for any of the sequels. While Dewey didn't commit any of the murders in the Scream movies, there's a persistent, popular fan theory that states he was a secret third killer in the original movie, but managed to get away with his part in it. Given how sweet-natured he is this seems doubtful, but maybe the upcoming fifth movie will revisit this idea.