The Killer is the unnamed antagonist of the infamous urban legend "Aren't You Glad You Didn't Turn On The Light?" - the legend is considered a modern day ghost story and the killer is never revealed but the concept of the story has lived on in horror as one of the more iconic urban legends and variants of the story appear in many media.
In the context of the story varies depending on the teller (common to all urban legends) but the basics always remain the same : a girl shares a room with a friend and arrives late one night, not wishing to disturb her friend she opts not to turn on the light when she enters the room and makes her way into bed.
Awakening the next morning the girl soon finds, to her horror, that her friend lay dead in the bed next to her and a mysterious killer had written, in blood, "Aren't You Glad You Didn't Turn On The Light?" on the wall : a chilling conclusion being that while the girl made her way to bed (in the dark) the murderer was in the room and remained their for some time after, presumably leaving once the girl had fallen asleep and leaving the message as a sadistic warning of how close she had came to death herself.
The killer's identity and motives are never explained, in keeping with most urban legends.