It predates Halo ...
Essentially, PC gamers constantly talked about upcomming games that had the potential to unseat Quake 1 and 2's multiplayer dominance as Quake Killers. Eventually Counterstrike, Team Fortress, Unreal Tournament and America's Army came along and Quake wasn't that dominant anymore and the concept of a Quake Killer died out.
The term entered into the console world mainly through discussions on BBS systems and the early internet and most big games (Zelda, Mario, Final Fantasy, etc.) have had games refered to as their 'Killer'. The reason Halo-Killer has become so dominant is because (durring the previous generation) the biggest dick-measuring contest between consoles was exclusive FPS games, and no exclusive FPS received the critical acclaim or sales success of Halo.
