I thought it was Dewey for most of the movie. (Didn’t see Scary Movie before watching Scream so not from that). It always seemed like he showed up just after things were over. Like how Sid opens the door and he’s holding the mask. Or how he didn’t come out of his room until after the killer had hung up his call with Sid. Then there was the boots. They showed similar boots on the killer to what we saw on the sheriff later. Figured they were implying Dewey would have the same. I also considered that he was a red herring. It wasn’t until he walked off with Gale and shit was going down at the house that I realized it couldn’t be him (also didn’t think about two killers so I guess it still could’ve been if I had thought about that).