RE: I think of Aliens...
Horror films involve elements of our nightmares. Not personal ones but collectively - a killer of our own creation; animals that want to feast on us; being trapped with someone/something that wants to hurt us; the dark; our phobias. These tropes make up horror films. These are visceral fears.
Science Fiction typically includes its own set of fears wrapped in allegory. But these are more cerebral - being replaced or misunderstood; fear of the unknown or the future; fear of choices made. A Sci-Fi/Horror film should both tell a cautionary tale with visceral fears to cement the tale itself. The Twilight Zone went from Sci-Fi to Horror to Sci-Fi again, sometimes in a single episode.
Aliens, Starship Troopers have "us" entering foreign lands walking tall, so to speak, and then finding ourselves overrun by something we've no respect for. Being overwhelmed IS a nightmare people have constantly.
