Yeah, sci-fi definitely mixes with horror nicely. The vast emptiness of space with unknown alien life and all that. Or man's search for knowledge getting him into trouble.Really when you look at it I would say the majority of the best horror and quite a high percentage of the best sci fi over the past 40 years has had a similar mix...
Alien, The Thing, Videodrome, Terminator, The Fly, Aliens, Predator, Scanners, Existenz, Pitch Black, 28 days Later, Sunshine, Moon, Under The Skin, Ex Machina, etc
It seems to be a kind of unspoken idea post Alien that if you want to make more ambitious horror you go the sci fi rather than the supernatural route, the only exception that comes to mind for me is Angel Heart.
Somehow I read that as "Jews In Space"...
And that settles it for me. That means it's a horror movie.
It's just a very very good, non traditional horror movie. Just like Jaws.
Jaws and Alien are my number 1 and 2 movies of all time respectively.
I have never thought of Jaws as a horror movie.
I'd call it a thriller. A closely-related genre, but not the same.