Well, they wouldn't have been a killer yet, would they? Only attempting to murder around eleven people in the library at that point.

[He's going to hum, and gesture over at where Vanis and Homura are talking about the lantern]

She died in the hedge maze, and there's the shattered lantern on the stairs. Maybe the killer held the lantern and dropped it while on the stairs. If Siobhan was chasing them, they got her to follow into the maze where they managed to get behind her and stab her?