Gears of War will not be a halo killer because it isn't the same type of game. It's like saying the PGR3 killer will be Rallisport 3. Gears of war doesn't even offer multiplayer games which Halo Fans dig.
I am going to say this once more, mark my words.
AFTER HALO3 GETS ITS RUN, UNREAL TOURNAMENT 2006 WILL BE THE NEW HALO FOR XBOX360.