After hearing the lighthouse keeper's assistance rant about seeing a stranger moving in the dark even though there's no one else on the island, you say goodnight to the lighthouse keeper and go up to your room.
The door is unlocked; you remember locking it. Nobody's inside, but there's a strange smell.
You follow the smell to your bed, and pull back the covers to find the lighthouse keeper: dead for hours, carefully dissected with his organs neatly arranged on your pillow.