A man said to the universe,
"Sir, I exist!"
"However," replied the universe,
"That has not filled me with
A sense of obligation."

-- Stephen Crane

That sense of cosmic contempt is what makes Alien tick. The universe is not simply indifferent to life but outright hostile to it. On top of that, instead of banding together in the face of this, many men elect to stab each other in the back. Alien manages not only to be about this monster that consists mostly of teeth and corrosive blood, but also the contemptuousness of one's fellow man. The alien isn't even the real guilty party: it's just a catalyst for other people's fear, greed and cowardice.