Event Horizon, and of course, Alien.
It's a similar line of debate as what always seems to happen regarding the difference between a thriller or a horror movie. Sadly, horror is still often looked down upon by the mainstream, with studios only too happy to try and court awards season by branding a particularly cerebral horror film a "psychological thriller." For examples of that, one need only glance over at Get Out.
While this kind of argument can be fun the results can never really be quantified, as what qualifies a film as a given genre can often be subjective. That's especially true of horror, which is as much about evoking a fear response as it is any particular plot element. Still, whether Alien is horror or not is by no means a settled matter.
Is Alien Sci-Fi or Horror? Twitter Weighs In
The big fight about whether Alien is sci-fi or horror - a question oddly predicated on films only qualifying as a single genre - began with a question posed by horror can't be set in space. She reasoned that "Horror is predicated on the fear of the other, the unfamiliar in the world as we know it – space, we already don't know it." Needless to say, the assertion horror can't be set in space drew some fire.
At that juncture, what had been a fairly one-sided debate turned into a contest to see who could most succinctly refute that opinion, pointing out multiple examples that disprove the idea horror can't be set in space, including slasher sequels Alien qualifies as horror, or is instead sci-fi. Most audiences will likely be content to call it both. Thankfully, the Twitter debate surrounding this issue occurred (mostly) without personal attacks or insults being in play, which sadly often can't be said about arguments surrounding films on social media. At the end of the day, if a movie has a rampaging monster murdering terrified humans, it's probably horror. Probably.
