Since Google didn't come up with anything for "Killer7 natural born killers" I figured I would post about it here.
Natural Born Killers has a surreal comedic presentation, a lot like Killer 7. There are also some specific things that caught my attention:
1) In the movie (Natural Born Killers), a cook stares down a bullet speding towards her forehead. In Killer7, Mask De Smith headbutts a bullet in a restaurant. http://video.google.com/videoplay?docid=-8442017913201418668#
2) Friend Travis wears T-Shirts that have various comments on them. In the movie, a projector projects statements onto the main characters' chests in the scene with the native americans.
3) There is a comment, in the movie, that a psychiatrist makes about how he doesn't trust anything women tell him. In Killer7, there is a russian roulette game with a climactic catchphrase of "...all women are the same".
I admit that the last one is a bit of a stretch. If you watch the movie, the aesthetics are very similar, and I'm surprised that I can't find any corroboration of it online.