I know, I know — Greg Dewey confesses to shooting Cassandra and is found guilty. But despite his confession, it's important to note that The Society never actually shows us who murders Cassandra. In the scene, Cassandra is ambushed by an unseen assailant with a gun when she's cleaning up after the school's impromptu prom late at night. She briefly begs for her life before being shot twice in the stomach, and is left to bleed out alone on the sidewalk. Yes, it could have actually been Dewey, but not having seen the shooter we can't be 100 percent sure.
If you ask me, the show is saving a big reveal for its potential second season, when we'll learn that Cassandra's murderer was actually someone else all along (*cough* Campbell *cough*). If Allie or the members of The Guard who killed Dewey find out they killed an innocent person, it could be enough to destroy alliances and shatter any semblance of rule within the town.