A note from the creators (spoilers? I guess):
This is late, but something we probably should have mentioned earlier.
There is no set murderer in the game, as the murderer is supposed to be ambiguous. The reason why we chose to do that was because we wanted the player to focus more on interacting with the other characters and learning who they truly were, which was the idea we created our game around. When someone asked our writer about who they believed the murderer to be, they responded with "I honestly haven't thought about that.".
The player is free to imagine the killer to be whoever they like.