Red herring. As we know, the Red Devil is also the school's mascot, and the costume was seen in the barista's dorm room closet. Therefore, any student (or faculty member) could have the same costume, but that doesn't mean they're the killer on campus. Boone faked his own death with the help of someone wearing the Red Devil outfit (maybe it's the barista or Chad, who knows). There might actually be two killers, Boone being one of them. This reminds me of the original Scream movie with Skeet Ulrich and Matthew Lillard working together to terrorize their neighborhood.