Well, an entire human civilization was overrun by xenomorphs and then blown up, according to AvP's prologue.Weyland-Yutani fucks things up because of greed.
Yajuta Hunters fuck things up for honor.
So, if you weren't stuck to a wall somewhere with an alien horror moving around in your chest (which'll soon be outside your chest), you were vapourized by an alien nuke because the aliens were too fucking incompetent to kill the other aliens they were hunting for sport/rites of passage.