Without circumstances known, a victim given, weapons and motives provided, any and all speculation is of the wildest sort. The question of each is whether or not they are a murderer, but shouldn't it be "could they become one"? Ane we still do not know what role this Black Shuck and the paranormal shall play. Arondight? A haunted island? A mysterious castle and a large student body? I sense closed rooms on the horizon, in a setting where we may not know the rules, the very laws of nature themselves...
Prepare for trolling, awesome, and an epic project full of mystery, I say! But let's hold off on judgements just yet... After all, any of them could be a murderer.
Or all of them.