It wants to know how much we know, it wants to learn from us, it wants to force us to rely on each other, it wants to incapacitate and distract us as well as those who might help us, it wants to punish someone for something, or it's doing it for the kicks. Those are my best theories. Oh, and it could be some sort of program run wild, but that sort of thing generally isn't this spiteful. Still, best not to discount it until we have proof to the contrary.