I still can't explain why the Xeno - the most vicious hostile agressive creature - wouldn't care less about a victim right in front of him (yes, for me it's a he).
He was very quiet at the ending scene, as if he was wasted, tired, trying to rest...
Was he sick? Or hurt, even?
Better hypothesis that just ocurred to me: was he at the end of its life? We never saw a Xeno lasting alive enough to know for how long they live. They maybe have just a few days to live! Hence their hastiness of wasting humans and nest them (to impregnate and ensure continuation of the species)...
A normal alien can easily live decades, or more (except when there is no queen, and they are capteured, see Aliens: Labyrinth). In the book (or comic, if you have it) Dr. Paul Church makes a lot of tests about the aliens, there you can find your answers.
I always assumed it was a "resting" time. Eat, sleep, attack, repeat. :D
Probably it was only luck to Ripley.
And it is not properly true that warrior aliens live for so long. Only queens can live for decades, but warriors don't last many days, probably a week or a month. The rate of growth of a single alien is surprisingly fast which will be responsible for a production of huge amounts of energy. Being that said, we can assume that aliens grow fast in order to become efficient killers very quickly, but they don't last long, so the queen can create more eggs, with more facehuggers, to parasite the humans that are captured by the alien warriors.
Well, in the end of Alien: Out of the Shadows, there are multiple aliens who survived the explosion, however, they return in the book, Alien: Sea of Sorrows what is take place hundreds of year after the first book.