Not that I actually believe this theory, but just to throw it out there...how about a demonic invasion/infestation. What's the name of the leader of our main group of warriors? Michael. Yeah, be shocked by that revelation. Maybe an invasion by extradimensional forces would work better. Perhaps Bill Roberts wasn't really schizophrenic, but was into some really weird stuff? What if all the tatoos weren't just for show but held some purpose? Could the main location of the gate in L.A. be the arena and that's why they drag humans to the middle of the rink?

Really I tend to lean towards some kind of sinister, Umbrella Corporation style, genetic experimentation, gone wrong type of thing with We're Alive.