Or tried to kill. This is only about the original Scream because I don't care enough for the sequels.
Steve Orth & Casey Becker: Both killed by Stu.
- Stu had more personal motive to kill those two. (Casey dumped him for Steve.) The only motive Billy may have had is to throw Sidney mentally off, but for that anyone could do and he could easily let Stu do the dirty part. I'm quite sure it was Stu's idea to pick Casey and Steve as the victims. Also, see the look in Casey's eyes when she removes the Ghostface mask to see who's butchering her. It is a look of regocnizing the person and regret/shock. Whereas she may have known Billy, that look messaged of someone she had hurt: Stu.
"I didn't kill anybody." "Nobody said you did." "Thanks, buddy."
That isn't a hint but just Billy trying to calm Stu down before he'd slip out something crucial. As it was true, nobody has said that Stu killed anybody.
Who was on the phone? Basically could've been either one but most likely Billy. The voice was aggressive and inpatient much like Billy, whereas Stu was more goofy and "relaxed" and likely wouldn't sound so self-controlled even when angry.
Attack on Sidney: - Stu.
The Ghostface stumbled quite a lot (which suits a goofy type of a guy), and there is no way Billy could've made it out and around the house and up to her window that fast if he was in the costume. Which leads to the fact that this was never a real murder attempt but just to have Billy cleared from the list of suspects by the police and have Sid feel guilty and something for them to make up.
Who was on the phone? Obviously Billy. The aggression again, especially in the bit about Sid dying just like her mother. Billy was the one with dripping hatred of Mauteen Prescott.
[[The call at Tatum's house: Definitely Stu. I doubt they leave arrested murder suspects in complete privacy at the police station + anyway the call had to be traced and had they traced it back to the police station...Plus, didn't they say that all the calls were listed to Neil's cell phone account.]]
The school toilet attack: Stu or just a prankster.
- Assuming Sid went to the toilet immideately after storming out on Billy in the hallway. There is no way Billy could've snuk into the girls' toilet after her, so Stu (or a prankster) had to be there all along. I'm still sating it was actually Stu as a part of their plan to make sure the school would be closed down until further notice and they'd have a reason to throw the party.
The principal: Stu or Billy, really could be either of them.
- Stu is the only one we see on screen around that scene but it doesn't really mean anything. He could've gone anywhere after convincing the girls to join the party. I'm gonna say Stu again for the sake of the final result and because he was the "thrill-killer" while Billy was mostly looking forward to the final act.
Tatum: Billy.
- Stu just knowingly sent her to her death. Stu was the host of the pary and thus couldn't have left the party, dressed up as Ghostface, murders someone, undressed and returned as if nothing had happened. Especially as he was the little goofy thrill-killer who got easily carried away. Also, as Billy pops up at the front door he gives Stu a look that might say that Tatum's "taken care of". Billy seemed to need a personal reason to kill, well he had one: Tatum had insulted him and his penis. ;D
I'm sure they intended to kill her anyway as they were aiming at being the only survivors left for dead, but Tatum just happened to say something that made Billy want to do the deed.
Intending to kill Randy: Beats me. I'm gonna say Stu, but I guess most officially it would be Billy.
- The film makers state that in the costume was Skeet (as in Billy's actor), but does this mean they meant Billy to be in it, is unclear. Would they risk it by Billy getting up and possibly Sidney somehow stumbling back into the room and find his body gone when Stu was already in the costume and all they had to do was to chase Sidney alone back into the house? For that reason I believe it was actually Stu in the in-story matters regardless of which actor was in the costume. But then I don't remember why it was Skeet, only that it wasn't said to be about the character.
Kenny Could be either. But I'm gonna say Stu.
- That is if Randy was almost-killed by Billy then Kenny was killed by Billy unless the 30-second-delay wasn't relevant and the other one came from somewhere else than the house. I'm still prome to believe it was Stu chasing Sid all along and committing the kills during it while Billy was playing dead upstairs. Oh, also because Billy said to Stu "I thought she was dead!" and Stu answered in a defensive tone "She looked dead, man." It gave me the impression that Billy scolded Stu for not making sure Gale was dead after Stu killed Kenny and placed him on the roof of Gale's van and so on.
Dewey: Stu or Billy but I'm gonna say Stu.
- Again because I think Billy had to be playing dead while Stu tried to drive Sid back into the house. (And again a victim wasn't checked to be really dead as Stu had made the mistake before.)
Attempting to kill Randy: Evidently Billy. I'm guessing he didn't know Randy was a virgin and thus not supposed to be killed, whereas Stu did know and thus didn't kill him in the living room (hence, the scene where it is Skeet in the costume but in my opinion Stu as the character. Hey, this actually supports my theory that Billy played dead upstairs while Stu killed people.)
RESULT: Sarcastically Stu killed more people than Billy even though Billy was the mastermind, the leader, "the main killer". I believe all that really mattered to Billy was to get to kill Sidney by his own hands, so he gladly left the lot during the plan execusion to Stu who, again, was the thrill-killer who needed no other motive but the thrill of the kill.
And above all, I'm convinced Billy killed Sidney's mom. No way in hell he'd let Stu butcher the woman after what she'd done to his, Billy's family. I'm also convinced he was the one to think of making the movie which would end with the death of Sidney. Because they never gave a seperate motive for the murderous rampage but Billy only mentioned why he killed Maureen. Apparently, the kid blamed the whole Prescott family - happy to frame the father and kill the daughter for the mother's sins and wanting to revenge them by his own hand.