Someone who overuses a joke so horribly that not only does it make entirely unfunny it also annoys everybody. Or someone who steals a joke and uses it so often it ruins the joke to the annoyance of the creator, sometimes causing violent outbursts.
Hear the one about the chicken crossing the road? "Who hasn't? The joke killer has been using that one 100 times a day for the past month!"
9/10 math teachers and English teachers
Damn dude, my math teacher, Mrs. Bowman, is such a boner killer
A fandom so large and toxic, it can take other fandoms and assmiliate it into their own by means of replacing the old fans with their annoying and toxic fans.
Friday Night Funkin' was the first ever fandom killer, as it was populated by nothing but immature fatherless children and pedophiles, no exceptions. The Fandom had the powerful ability to create mods, which meant they could make mods based on other fandoms. If the mod got traction, then children would flood the fandom and push all the old fans out, assimilating their fandom and the thing they loved so much, and now it would be only known as another subgenre of the FNF fandom.
Guy 1: What's *insert thing*? Why does everyone keep talking about other things with it?
Guy 2: It's some weird game. The fandom got so large it became a Fandom Killer. It took my small fandom.
The person/friend that sleeps with the bartender, thus making future visits to the bar uncomfortable and not as enjoyable as it used to be because of their weird dynamic
Stop sleeping with all of the bartenders, you're a bar killer
A game that is labelled as WoW Killer is usually a game that commits suicide 1 month after.
"Have you heard of the new xy game? It's the new WoW Killer!"
*1 Month later*
-xy Game is dead.
Deer hunter who enjoys stalking and killing deer
Check out the way that guy smiles while chopping off the head of that deer, dude is for sure a Deereal killer.