What's interesting is the dichotomy between the humans and the xenomorphs. The Xenomorphs are primed for survival - considered the "perfect monsters" due to their defensive and offensive capabilities (acid blood, deadly tail, strength+size, etc.) (see: Sun Tzu's shuai-jan). And yet, in (almost) every movie (Ripley commits suicide in the third movie, but I guess she gets cloned in the fourth one, so I'm not sure that counts), there is a human survivor. Is it because the motivations are different? Fighting for the individual vs. the collective? Ripley and the other main characters tend to be isolated while the rest of the crew gets slaughtered, whereas while the Xenomorphs can be alone (like in the first movie), they are fighting for their species, as they can't procreate without the face-huggers. Very relatable to the Japanese Battle Royale movie (which I'd give a 6/10, because while I enjoyed the killing, naturally there wasn't a last man standing, rather a last couple who escaped together...BORING!), where we see how different people behave in different contexts. Survival of the fittest. What people will do to save a life (either theirs, or someone elses). The point of this is - Alien isn't about the actual aliens, it's about humanity! The Weyland-Yutani Company, for instance, knew of the destructive nature of the Xenomorph, and instead of pushing for extermination for the good of the human race, they looked to take advantage of the aliens - to use them as a bioweapon. Characters like Ash and Burke remain loyal to the company, willing to expend anyone and anything in order to retrieve the Xenomorphs (via face-hugger specimens), despite them being in the same boat as the other characters. Thus, not only do the movies encapsulate human emotion, irrationality (such as when Dallas overrides Ripley's call for quarrantine and brings Kane onboard), and tenacity, but corruption and greed as well. My favorite characters are Ripley (naturally) and the synthetics: Ash, Bishop, David, etc. And Meredith Vickers (who surprisingly isn't a synthetic herself).
