(n.) A person who, knowingly or unknowingly, repeatedly destroys or "kills" otherwise funny threads on public social networking sites with a single, stupid comment.
Guy 1: "Hey, man, look at that idiot's comment; the fourth one down."
Guy 2: "Ugh. I hate when people kill hilarious threads like that. What a douche."
Guy 1: "I know, right?? That guy's a real Threaddy Krueger."