Try to be really attentive and careful here, cause in this game you are going to kill them all and stay alive. Watch out! They are already here! So, the main thing is to survive and clean this world from all these creatures. Just enjoy the game.
Just imagine that you are one of the many prisoners captured by the aliens. They will brainwash you and turn you into a mindless soldier, you only have one goal just to escape. Just imagine that most humans and animals have already been wiped off the planet, the ecosystem has been altered to suit the needs of the aliens, and the air has been made unbreathable for most earth creatures, including humans. So, if you wanna to save the whole world, then you can do it! Everything depends on your skills. Just have fun.