
In real life, often just a knife or gun will do.
But in Horror movies, implements of death are sometimes required to be a little more creative, a little more unique, as they often become trademarks of the films (the fingerknives in "A Nightmare on Elm Street," the Driller Killer in "Slumber Party Massacre," the floating steel ball in "Phantasm," etc.).
In many movies (particularly slasher films), the killer will use several different weapons to dispatch his victims (in the Friday the 13th series, it becomes a game to see what idiosyncratic tool Jason Voorhees will wield next). Since we only list one weapon on our page, we generally pick the one that was most memorable, either from repeated use or most unconventional.
Sometimes you just have to see these movies to know what we're talking about.