I am also sure you could play Killzone 3 and not play Killzone 2. I would suggest playing KZ2 though, but I did not play the original on PS2 and I followed everything just fine.
I am sure they will provide some back story.
iPhone = Great gaming device. Don't agree? Who cares, because you're wrong.
Currently playing:
Final Fantasy VI (iOS), Final Fantasy: Record Keeper (iOS) & Dragon Quest V (iOS)