These two girls were roomates in a college dormitory. One night, one of the girls went out on a date, but the other was really tired so she decided to stay home and go to bed early.
Around midnight, the girl who had gone out realized she had left her wallet in the dorm, and she needed it for some reason or another, so she decided to go back and get it. Her date waited in the car outside while she ran in. She remembered that her roomate had been exhausted, so she didn't turn on the light for fear of waking her. She just snuck in, grabbed her wallet off her desk, and snuck back out.
She came home early the next morning and found her roomate still in bed- dead, her throat slit, blood everywhere. And written in blood, on the wall above her bed, was the question, "Aren't you glad you didn't turn on the light?"