My theory: AGM is a company in Mars that makes/kidnapps aliens and unknown creatures to do experiments on them, freedoomguy is a marine who broke some AGM rules and then he become a prisoner but then decided to escape but then made a big fat mistake causes the monsters and every creature to get out all the AGM scientists were killed by them and then freedoomguy decided to do anything to kill the aliens after he cleaned up mars he noticed that he isn't complete the monsters already went to earth freedoomguy decided to go to the AGM labs to portals that makes him go to earth and then he succeeded but where he landed? To the cities that has millions of monsters he cleaned all if them but one of the cities has lots monsters that freedoomguy can't kill them by himself he then decided to nuke the whole city after he finished to kill those idiots on earth he finds a portal enters it he goes to place that is so hot and it has even more monsters freedoomguy continues to sacrifice his life and then finds large skull spawns monsters from his brain kills it and then kills all of it's monsters freedoomguy decideds to hide from earth so no one can know who saved them no one will ever know how did all of this happened