In the conclusion of the first game the locust queen mentions that humanity has brought this war upon itself. I first thought that the over mining of imulsion pissed the locust off while that is still possible the events at New Hope research facility point to something else. After reading the collectibles of that level and the dialog I came to this conclusion:
The locust are experiments gone wrong.
But under their pissed off nature they may only be vengeful.
The fact that New Hope was only declassified recently points to that being the chairman's dirty little secret; he knows the origins of the war. Why Delta squad was sent there, was probably desperate need of intel. They say that they were trying to cure a sickness. They were holding people against their will, a bit like Island. They did tests on them to alter there genetics to make a super-species. I think they where trying to cure cancer with imulsion-therapy, thus mutating the victims into locust and torturing them horribly. It might also be to create a superhumain army in the pendulum wars.
Niles mentions his incursion in the mountains, he probably brought the original locust with where they escaped into the hollow, pissed off and wanting to revenge on humanity for there suffering explaining what they did to Tai, Maria and all the other prisoners. The sires are the left over locust who without supervision since Niles probably never came back from the mountains, continued to mutate into what we chainsawed through.
Another question was why the Locust queen seemed humain? I believe that she was a New Hope employee who freed to locust. After reading Dr Stephanie Zimmer's journal it might be remotely possible that she is the queen, but Zimmer seemed to rather leave than do something.
To sum it up simply; the locust are test subjects of a crazy scientist, who escape because of an employee that now leads them. They are pissed off at humanity and want to make them feel like them.
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Catch me playing Far Cry 2 ranked matches on Xbox 360.
