How about this: The original xenomorphs were intelligent and had a culture; a culture of power and domination, one decided to become more powerful and mutated itself into becoming the classic xenomorph; the perfect organism. This ancient civilization eventually succumbs to self-destruction after much conflict, with one notable poor sap succumbing to deadly cargo and crashing on LV-426. All that remains are the "engineers" - a creation of the ancient, intelligent xenomorphs - isolated on a planet to carry on the legacy. What was left of the xenomorphs was broken down into the accelerant, and locked away forever, with a covenant made never to re-animate it. Someone broke that covenant...