Someone who is not into what everyone else is doing.
"Judy, when you mention your yeast infection when I'm all hot & bothered, it's a real fun killer."
"The cops showing up when we are chucking bottles off the balcony is a real fun killer."
Person 2: That's not true, I work in a dairy factory and it was discovered that milk does not cure a hangover.
Person 3: Goddamnit, Person 2! It was a funny story. *sigh* You stupid Fun Killer