According To This Theory, Dewey Is The Serial Killer Mastermind In 'Scream'

An interesting clue hinting that Dewey is Billy and Stu's puppetmaster is that Dewey is harmed in the same manner as the teenagers, who knife one another to feign their victimhood. Billy and Stu explain that if they both have enough shallow punctures, no one will suspect they're actually the culprits. 

Similarly, Dewey has shallow knife punctures on his back and a broken arm, but nothing life-threatening. Why - after they literally rip apart their other targets - would Billy and Stu take it easy on Dewey, unless he was also involved in their plot?

One of the many running themes of the Scream franchise is that Deputy Dewey is absolutely no help in catching masked offenders. In every film, he is either knocked out or harmed, or he sleeps through every important moment. Some fans, like Redditor /u/zionhyome, believe this is all purposeful:

He is always either incapacitated or otherwise busy during the final showdown. This is the big one, for me. During this time, he could be acting. If he's in on it, he's nothing to lose with either side. He could be waiting for them to finish, so that he can take the side of the one that wins.

Numerous clues hinting towards Dewey's involvement in the Ghostface incidents are sprinkled throughout the first Scream film. Some are massive, while others are more subtle. One of the more subtle hints, as many fans of the film have pointed out, is Ghostface's police-issue shoes - the same ones worn by Dewey. 

Modern-day online research easily reveals the tactical wear that Woodsboro officers use to suit up, but in 1996, internet shopping was hardly so thorough. Insular information was more than a few seconds away, and to become familiar with police footwear, you'd have to know someone on the force. As such, Dewey likely gave Billy and Stu police boots in order to further mask their identities.

Sidney Prescott lives in the middle of nowhere. Her father's house is surrounded by woods and rolling green hills, and in the impressive helicopter shot that ends the film, it's clear that no one lives within a mile of the family - so how does Deputy Dewey reach her house so quickly after she tries to make an online call to 911? 

When Sidney is being chased through her home by Ghostface, she tries to call 911 from her computer before the intruder stops her. She escapes his grasp and reaches the front door, where she finds Dewey waiting and holding the Ghostface mask. Fans are suspicious of Dewey's quick arrival - while he may have simply been watching out for his sister's friend, he also may have given Billy a ride to the remote home.

Something many fans find strange about Dewey is the way he mourns the loss of his sister, Tatum, after her neck is broken by a garage door - that is to say, he doesn't mourn her at all. Some believe he merely saw his own sister as collateral damage in an obsessive conquest for her best friend, Sidney.

One theory about Dewey's puppeteering in the Scream films says that he never mourned Tatum because he was implicit in her end. Whether or not he gave Billy and Stu explicit instructions to take her out is unclear, but if he was behind the slayings, he had to assume his sister would eventually become a target. 

In most of Scream's harrowing scenes, Dewey arrives immediately before or after the appearance of Ghostface. Whether he's popping up at Sidney's house right after she's set upon by Billy or if he's at the school where the principal was eviscerated, Dewey is always nearby. 

More so than Billy or Stu, Dewey is akin to Jason Voorhees, Michael Myers, and the lesser slashers that followed in their lumbering footsteps. Like those characters, Dewey is nearly omnipresent and is always exactly where you don't want him to be. Whether he's taking a hands-on approach to the slayings or orchestrating them himself, his nearly constant presence at the scenes is proof of his involvement.