Like if you've ever played Starcraft, the Campaign has you playing as each of the three factions for various parts. It's a neat feature actually, giving you insight as to why they do what they do.
The Locust, for example, didn't invade the surface world simply because they felt like slaughtering humans. They did it because they had no other options to escape or destroy the Imulsion. Having a figurehead like Queen Myrrah constantly telling them that
they hate humans, so that they begin to believe it themselves, makes them better soldiers. In any war, the conflict is not about the individuals who play it out, but rather their governments who instigate it.