The Third Killer is just a theory and never proven. The killers had partners in Scream 1, Scream 2 and Scream 4. However, after their revealing, many questions have gone unanswered, as it appears that both killers could not have been able to commit certain acts, either due to their presence being known or inability due to what we may know about them. As a result, many fans believe there may be a third killer throughout the Scream franchise.
- Who framed Cotton: Both Billy and Stu admitted to framing Cotton. However, Sidney claims she saw Cotton leave wearing his jacket, which is later revealed not to have been Cotton. Neither Billy or Stu could have been the framer for two reason: Billy, though having long hair like Cotton's at the time, was too short and skinny to be have been the coat-wearer. Stu, though the same height, like Billy was too skinny. Cotton was a muscular large man. A possible third killer who resembled Cotton from behind with the same height and muscle appearance could have been responsible for this. It's possible with Roman Bridger claiming to have killed Maureen in Scream 3, that it was Roman who was wearing the jacket. Though he's not as tall as Cotton, his age and body built is closer to that than the obviously younger and skinnier Billy and Stu.
- Opening murder: It is true that both Billy and Stu were present during Casey and Steve's murder. But evidence show that another killer was present. When Casey finds out Steve is tied to a chair, Steve looks both to his right and left which indicate that both Billy and Stu were hiding on both sides of the house. Yet, not long after his death, with both killers having to still be in the back, somebody rings the door bell. When Casey is hiding from the killer after the chair is thrown, two killers are seen. Yet the direction of the killers and positions of the house shows that more than two people were inside the house. Who caught Casey at the window? Some say Billy while Stu crashed into Casey from a window to stab her moments later. But if that was Billy, he'd have cuts and bumps from smashing his head into the window and from Casey whacking him with the phone. He does not show any sign of injury in the next scene when he sneaks into Sidney's bedroom. Also even with both Billy and Stu, the time between the killer dragging Casey to the tree and her mother finding her hanging is too short for just the two of them to quickly get done. A third person was needed to achieve this quick action.
- Who was stalking Sidney: While it's possible that Wes Craven was messing with our heads in the scenes where Sidney and Tatum are stalked by the killer in broad daylight, it's there plain and simple. A killer was stalking Sidney. However, in between the two stalking scenes both killers are present at the video store with Randy. If that's the case, who is stalking Sidney in the killer costume?
- Dewey and Sidney's climax attack: Most fans believes that Billy was the one who stabbed Dewey and attacked Sidney during the final chase scene before the revealing scene. Yet Billy wouldn't have had time to disappear, sneak into the house, go upstairs, ditch the costume, and reveal himself. And it's clear the killer in that scene wasn't Stu. So a third killer had to have been the attacker in this scene when neither of the two killers could possibly have had the time or ability to do that.
Third killer theory: Roman Bridger. In Scream 3, he took so much credit for the events of the first movie that it's possible if there were a third killer that it'd be Roman. At the time, Roman had no actual plan other than executing his own mother using Billy and Stu. He let them make their own plan, but still believed they could be caught and decide to run as soon as it's made clear they were going to be killed.
- CiCi's Caller: When CiCi and her roommate are talking, they get a call from the killer claiming to be CiCi's boyfriend Ted. But when the killer is seen sneaking in, there is no phone, and we clearly hear the killer talking when this happens. It is believed that Mickey was the one who killed CiCi. But it's known that Debbie wanted nothing to do with the new killings as her targets are Sidney, Dewey, and Gale. Besides the killer's attitude towards CiCi is not like Debbie's. Debbie would not waste her time with CiCi. So it's possible a third person was the caller while Mickey did the killing.
- Projector Room: When Dewey and Gale are in a classroom looking through videos to see if any clues can be found to identify a possible killer, they're interrupted by a killer starting a video in the projector room. However, after Dewey finds it empty, another killer sneaks up behind Gale from under a desk which we later learn is Debbie Loomis. This killer couldn't have been the one in the projector room. Also between the distance and time from their attack to the cops attack, it could not have been Mickey either. So a possible third killer was involved in Dewey and Gale's attack.
Third killer theory: There is no major theory as most questionable moments are answered. The two unanswered could just be moments not thought out well. But possible suspects are Joel, Sister Louis and Sister Murphy. Although neither have any motive against Sidney and the victims. Though we do meet a character played by Joshua Jackson. He appears to know a lot about horror movies. Why would a big named actor be cast without giving him more? Could he be a possible third killer whose role just never blossomed? It is known that script leaks showed that both Derek and Hallie were partnered up with Debbie. But these changes and their presents in each questionable moment exclude them.
In Scream 3, Roman is revealed to be the sole killer. (Originally, Angelina Tyler was going to be revealed to be the second killer.) However, there is still evidence for the third killer theory.
- Who killed Angelina: Immediately after Gale and Jennifer find Roman "dead" they run quite a distant and into Angelina. After realizing they're in danger, Angelina runs off in a panic and is attacked by the killer. However, between the amount of time of his alleged death discovery and the distance between his death scene and where Angelina was killed does not consist with the amount of time and distance Roman would need to kill her.
- Who is Where: John Milton's mansion is so large that even a person familiar with his home could still get lost or take some much time to get to certain places. The amount of time and distance between Tyson's death, Jennifer's death, and Gale's basement attack could not have all been done by Roman on his own.
Second killer theory: Mark Kincaid. Appearing right after the killer disappears, before even revealing Roman's death was faked, it is his police issue bullet proof vest that Roman is wearing at the end. This is fully supported in the subtext, including but not limited to notes in Kincaid's office, Kincaid's insistence Sydney be brought where he had access to her, and how Roman could already be waiting for Sidney in his secret room while another masked killer was still trying to break in. He was never caught, but subtlely alluded to his involvement at the end, while offering to show a movie for Sidney and her friends, implying, but not stating, that it was a scary movie.
- Who killed Rebecca: Both killers were present when Rebecca was murdered in the garage. However, both were not alone therefore could not have killed Rebecca. Charlie was with Robbie and Jill was at the hospital, most likely being watched. So there is no way neither could kill Rebecca. Also neither would mostly due to their age know how to disable a car nor be able to carry her up to the top of the hospital.
- Who attacked Gale: Neither killer could have done this as Jill was not around and Charlie was with Robbie. Though unconfirmed, some fans claim they have spotted Charlie in the crowd running in a panic when Dewey arrives shooting at the killer. If this were true, then this leaves Jill as the killer since she later exposed herself when she mentioned her and Gale's matching wounds. But once again, she's not in the position to do this as she was with Kirby.
- Roberts home: Neither killer could have been present during the ambush of the Roberts home. Charlie was with Robbie and Jill had been picked up by Kirby. Two killers are present as one is seen in the backyard but then immediately at the front. Immediately after Kate is killed, Deputy Hicks appears. Most fans have theorized that Hicks may have been a more superior and tougher partner for Jill and Charlie.
- Who is attacking Jill and Charlie: Near the end, both killers are clearly not in the position to be the attacker when both are simultaneously being "attacked." While Jill is under the bed, the killer goes for Sidney. This could be Charlie. But not long afterwards, Charlie is seen being attacked by a killer most see as Jill. However, neither killer would have time to switch places. So a possible third killer is attacking them both. It could be that while this third person is attacking Jill and Sidney, Charlie is hiding Trevor. After hiding Trevor, Charlie goes to be "attacked" by the third killer while Jill takes her time to prepare to reveal herself.
Third killer theory: Most fans theorize that Deputy Judy Hicks could be the third killer. She has the possible motive of being in love with Dewey, being jealous of his marriage with Gale, and jealous of Sidney's fame as she admits that she knew Sidney from high school. After Jill betrays Charlie to be the sole survivor, Hicks (who was not near Dewey when he called her to go to Kirby's) could have decided to abandon her plans to reveal herself to play hero to save herself from being caught.