I'll give you points for Alien allegory but it can also be seen as an invasive hostile racial vector that foolish Dallas (European city until recently) opened the airlock (city gates) to the foreign presence.
So it arrived in the craft, incubated and emerged to spread racial chaos and violence.
Only through Ripley, essentially the Sophia-type figure to purge the racial vector by any means than risk the core-racial structure of the homeworlds be infected. Hence the requirement to abort the enemy via the airlock and ships main engines to cleanse the threat completely...