As told by Whorton:
Two dormmates in college were in the same science class. The teacher had just reminded them about the midterm the next day when one dormmate let's call her Juli got asked to this big bash by the hottest guy in school. The other dormmate, Meg, had pretty much no interest in going and, being a diligent student, she took notes on what the midterm was about. After the entire period of flirting with her date, Juli was totally unprepared for her test, while Meg was completely prepared for a major study date with her books.
At the end of the day, Juli spent hours getting ready for the party while Meg started studying. Juli tried to get Meg to go, but she was insistent that she would study and pass the test. The girls were rather close and Juli didn't like leaving Meg alone to be bored while she was out having a blast. Juli finally gave up, using the excuse that she would cram in homeroom the next day.
Juli went to the party and had the time of her life with her date. She headed back to the dorm around 2 a.m. and decided not to wake Meg. She went to bed nervous about the midterm and decided she would wake up early to ask Meg for help.
She woke up and went to wake Meg. Meg was lying on her stomach, apparently sound asleep. Juli rolled Meg over to reveal Meg's terrified face. Juli, concerned, turned on the desk lamp. Meg's study stuff was still open and had blood all over it. Meg had been slaughtered. Juli, in horror, fell to the floor and looked up to see, written on the wall in Meg's blood: "Aren't you glad you didn't turn on the lights!"
As told by Jon Little:
I heard about a girl who went back to her dorm room late one night to get her books before heading to her boyfriend's room for the night. She entered but did not turn on the light, knowing that her roommate was sleeping. She stumbled around the room in the dark for several minutes, gathering books, clothes, toothbrush, etc. before finally leaving.
The next day, she came back to her room to find it surrounded by police. They asked if she lived there and she said yes. They took her into her room, and there, written in blood on the wall, were the words, "Aren't you glad you didn't turn on the light?" Her roommate was being murdered while she was getting her things.
(I've heard this several different times. Each time it was at a different university.)
Analysis: These are variants of a widely told legend given the title "The Roommate's Death" by folklorist Jan Harold Brunvand (see The Vanishing Hitchhiker, W.W. Norton, 1981). In every version of "The Roommate's Death" someone is killed right under the nose of an unsuspecting female protagonist, but because the lights are out, or the crime takes place in another room, the victim's body isn't discovered until later, usually the next morning. As the story is sometimes told, the protagonist hears suspicious noises but is afraid to investigate because she thinks it could be an intruder coming after her.
The "creepiness factor" is very high in "Aren't You Glad You Didn't Turn On the Light?" On discovering the body, the main character can't help but realize what a close call she's had. And the murderer rubs it in with a message scrawled in blood.
While the general form of the legend dates back at least 40 years (and surely more), it has a timeless appeal as a specimen of the "American adolescent shocker story," to borrow Brunvand's phrase. As he wrote in The Vanishing Hitchhiker,
One consistent theme in these teenage horrors is that as the adolescent moves out from home into the larger world, the world's dangers may close in on him or her. Therefore, although the immediate purpose of these legends is to produce a good scare, they also serve to deliver a warning: Watch out! This could happen to you!
As is often the case with so-called "cautionary tales," however, the warning is of little practical use to those who hear and repeat the legend apart from providing some degree of catharsis vis-a-vis the inevitable fears that accompany growing up and moving away from home.