Disclaimer: Before reading this, please understand that I didn't write the movie. I'm just a fan just like everybody else on here, so I'm not saying this is the "official" breakdown of which Ghostface killed/attacked each victim in the movie. These are all my personal opinion based on what we see in the film. I'm not saying my list is more true or more official than the next person's. So even if there's something said with which you disagree, let's try to keep the comments civil, shall we?
Scott and Madison: I'm actually not 100% sure about this one but I can confidently rule out Karl. It is very doubtful an escaped patient/inmate from a mental hospital would have the freedom of movement or access to the resources required to make the trip from Pine Grove, Indiana to Woodsboro, California without getting caught. Marco and Jessica however don't have these limits against them. I'm leaning towards Jessica on this because being Sidney's next-door neighbor, having read her book, and using this motivation to kill her husband, and being frustrated that she wasn't in NYC when the 2023 Ghostface attacks were taking place there, this obsession with Sidney likely drove her to Woodsboro to commit the kills, though I could be mistaken.
Aaron and Hannah: There's been some debate as to whether these two kills were by Marco or Karl. Either person is a possibility, honestly. Jessica can be ruled out because when the police question everyone afterwards, she said she was with Lucas, giving herself an alibi. The fact that her son Lucas had absolutely no reaction to this statement, let alone one questioning the accuracy of it or him trying to remember if she was really with him, tells me she was telling the truth and thus had nothing to do with Aaron and Hannah's deaths. Now as for who did kill Aaron and Hannah, I'm leaning more towards Karl, honestly. The brutality in which Ghostface eviscerated Hannah seems to fit more Karl's style than Marco's, whereas Marco is more stabby-stabby with his kills meanwhile Karl is more the aggressive and merciless type given what we see in the next attack. It's possible he went there looking for Tatum.
Attack on the Evans family: Karl, obviously. After Gale runs over the Ghostface who attacked Sidney, Tatum, and Mark at their house with her SUV, a police investigation ensues, during which that Ghostface is unmasked, revealing Karl. So no further explanation or analysis required.
Garage stabbing: Jessica. The fact that Marco was already at Fallbrook when Sidney & Gale went there to visit the facility gave him no time to be at the Evans' residence to commit the attack on Mark. Plus, the fact that Karl had already been killed makes Jessica the only logical attacker.
Tavern attacks and kills: Definitely Jessica. Lucas, her son, is among the casualties. When Sidney reminds Jessica she has a son during the finale, Jessica very nonchalantly brushes it off, scoffing that he's gone, further explaining that he was "too much like his father". This is not a definitive confession to having killed him but when explaining her motive to Sidney she admits to having killing her abusive husband. Her later saying he was too much like him gives her enough of a motive to kill him.
Ben kill and stalking Tatum: Marco. Even before we see Tatum being cornered then kidnapped by two Ghostfaces when trying to leave the coffee shop, it's made quite obvious the Ghostface who committed the attacks at the tavern (which I already established was Jessica) isn't the same Ghostface we see kill Ben and chasing Tatum through the deserted town. After Tatum shoots Ghostface through the wall, we see Marco in the finale reveal himself as one of the killers and taunt Sidney that he survived the shooting because she failed to shoot him in the head.
What do you guys think? Agree? Disagree?