As if I would ever admit that something as ridiculous as ghosts exist.
Betty’s death was an accident. The text hints that Betty likes to sleep around, or at the very least, flirt around. She left the party in order to met random guy X in the library where they were planning to fool around. They entered the library through the hidden passage. Betty, being too drunk, ended up up in an accident that killed her, maybe she slipped and hit her head on one of the desks in the library causing her to die. Guy X used a heavy book to smash Betty’s face in order to make it look like a legit murder and escaped through the hidden passage. Why he did it? Betty either proclaimed she wanted to break up before dying and he was pissed, or he was thinking illogically and thought destroying her face would help his case. There is no legit culprit in the mystery due to the death being an accident caused by Betty herself.