(conjecture) Xeno's are a Warrior Class

The Engineers are parasitic, sentient beings, bent on galactic domination, who have perfected biomechanics and nanotechnology. The xenos were engineered by them as a self replicating ground force that is practically unstoppable. The engineers are so aware of the deadly nature of this weapon that they take elaborate steps to ensure that the egg chamber is difficult to access and that the eggs remain in a perpetual stasis. The collision with the prometheus causes the stasis system to malfunction enough to hatch a facehugger which impregnates the pilot. The pilot attempts to use the ship's technology to remove the embryo, but he is too late and the chestbuster is born lethally, but the engineer kills it with his aborting mechanism before it expires. Shaw manages to transmit a warning after the ordeal with the engineers and xenomorph, but the company encrypts and corrupts it after they reroute Nostromo, so they can sell it as a distress call because they want to cultivate this weapon themselves. They are simply ignorant of the built in kill swtch that died with the Engineer.