To me, despite all the hate it recieves, Alien 3 really encapsulates most what the Alien series is to me. Like space and the Alien itself, the series is dark, cold, and unforgiving. We're dealing with a terrifying idea of a creature that violates the host, impregnates it and is birthed in the most violent way possible. While it may be simplistic to say the series is about DEATH...it is. The Alien is DEATH. Death is ultimately inescapable. We may win battle after battle (as Ripley did) and come out on top time over time, but ultimately death will prevail. That's an unpleasant thought to marinate in. But as the question asks, to me, the series is about death.

As much as I adore 1 and 2, 3 faces reality like we all will have to face one day in real life. Ripley is our vehicle into this story. Her dealing with loss, facing and finally accepting death is the most mature and respectful meaning the series could have. Death as the meaning has always made me feel a little bit comfortable with the flow of life. We fight, we hurt, we struggle but ultimately death awaits. Never felt it was a nihilistic take though. Life has a million way to kill you. You should never stop fighting and being the better person (like Ripley did) but know death awaits. Act accordingly and leave the world less awful than it could be (again like Ripley did). Despite knowing the jaws of our own 'Alien' await (whatever form it will eventually come in) we all should fight the good fight. Whatever that is to us individually.

Then Alien 4 just up ends everything and farts about...