Literally how it's read. Twilight Buzzkill. A harsh slap in the face to bring a person back to reality, to get out of Twi-Land. Usually a person has to tell the Twihard to get a life you Twifuckup, and that usually does the trick. Sometimes the Twibuzzkiller is just a big A-Hole, though. I'm not taking sides, here.
Twihard: I wish I was a vampire! Golly!
Twibuzzkiller: Are you kidding? There are so many things wrong with the book! I mean, first off, If the Cullens are so keen to the scent of human blood, how can they go to school around girls on their periods? You are stupendously-
Twihard: Just shut up, OK!!! You're such a Twilight Buzzkill!
(stalks off and pretends to be a vampire.)