What is Halo-killer?
1.
A very high-quality shooting video game.
Many say that Gears of Wars will be a Halo-killer.
See
2.
A console video game (a first-person shooter specifically) which is better than, or claims that it is better than, the games in the Halo franchise. Perfect Dark claimed this and failed, Killzone claimed this and failed. Call of Duty 4 is indirectly claiming this and might fail. No game has yet been able to claim the spot of being better than Halo, and to claim to be a halo-killer pretty much destroys any slim chance they might have had.
"Dude, Perfect Dark is gonna be a Halo-killer."
"No, it's not, you are wrong, asshole, Perfect Dark blows."
See