I think Ridley Scott saw it as a horror movie in space, period (keepin' it simple, stoopid I guess is a good idea), Sigourney Weaver, IIRC, has occasionally mentioned that is has shades of fear about biology and the body.
Hey, what do I know? I prefer Aliens anyhow because it has drop-ships, APCs, space marines, MG42s on steadicams and the line "I say we nuke the site from orbit."