Curious title, no? Contentious even. But, bear with me, as I give my thoughts on this...
Defining certain films as horror can be a tricky business (indeed, defining horror as a genre itself can often be laden with hardships and arguments), yet there are many films out there that a hell of a lot of people would agree are horror films, despite being marketed as something else. Films such as Alien (SF), Se7en (gritty crime/serial killer), The Silence Of The Lambs (crime/serial killer), Under The Skin (art-house/SF) and many more, promoted as one thing but sharing a common factor in that they scared, horrified or disturbed a good number of people. Now, a lot of this is subjective, down to interpretation and just because a film (or book) does this, doesn't mean it's a horror (Schindler's List horrifies me with some of its content, but I wouldn't call it a horror film). So there must be something else. I think it's a combination of intent, atmosphere and delivery - all of these films intended to scare their audience, their atmosphere's are ones of anticipation, dread and claustrophobia and regardless of their settings or the trappings of the film, would work just as well if the location/character roles were changed (to a certain extent). Alien would work just as well in a large abandoned house, the detection and police procedure in Silence... is largely redundant... Well, that's what I think.
Now, keeping that in mind, are the SF trappings of The Terminator integral to what is occurring on the screen? And you might immediately say "Yes!" But while I'd agree that they are more important than, say, Under The Skin taking place in and around Glasgow, I would also argue that they aren't all that important in the wider view. The drive of the film is the pursuit of a vulnerable person (in this case a female), by a seemingly unstoppable killer, who cuts an extremely violent swathe through anyone that gets in his/its way and is only destroyed/stopped at the end by the one surviving female... Sound vaguely familiar? It is my glorious belief that The Terminator (the first film only) is the most perfect distillation of what a slasher film should be!
Okay, bear with me some more. The first time this occurred to me was when watching Rob Zombie's Halloween 2. Now, I'm a massive fan of slasher films, the good and the bad (and there are so many bad ones...), but for a long time, I've felt that they are just, well...silly. Until Scream came along (and I love those movies, so no bashing), they were poor parodies of the genre, cringe-inducing comedy and bad effects, designed purely to off a bunch of good-looking Hollywood youngsters. Scream gave a bit of heart back to the genre, but was quickly followed by lesser and lesser copies. Then Zombie brought out his remake of Halloween, which I actually really liked, at least up until it started copying the original film. I liked the slow descent into inhumanity of Myers and his growth to the hulking monster he became. Then I watched the second film. And was blown away. By its brutality, by its total lack of cheesy comedy, by its all too believable central character as a genuine unstoppable monster...and it hit me. He's a fucking Terminator!
The more I thought about it, the more sense it made. It's even in the dialogue;
"It can't be bargained with. It can't be reasoned with. It doesn't feel pity, or remorse, or fear. And it absolutely will not stop, ever, until you are dead."
To me, The Terminator is the ultimate slasher antagonist. The only difference between it and accepted slasher films is that he doesn't use blades or knives, he uses guns. And the explanation for his rampage is scientific rather than grounded in some sort of psychological trauma or the occult. But other than that? It's a slasher. I'm telling you. I suspect the body count is far higher than most slashers, I'd far rather my chances against a Voorhees or a Myers than a Terminator, and even the violence seems bloodier; more vicious. So, what about intent, atmosphere and delivery? Well, I don't know for sure, but judging by the way the film unfolds, I'd say Cameron knew exactly what he was doing; he wanted to create a terrifying object in the form of the Terminator. The atmosphere is tense, brooding and full of dread; you never know if these people are going to survive (and against such odds, how could they?) and the delivery is like a typical 80s horror film; it has the low budget credentials, the analogue/synthesizer score and the borderland sleaziness inherent in many dark films of that era. Finally, it terrified me as a kid (admittedly, that might not be much of an argument as Mickey's Halloween Special scared me at that age too), and that image of the Terminator rising from the flames like a shining metal demon with its skin and flesh burned off will stay with me for as long as I live.
There are probably more things I could say in support (and I'm sure there's a lot that could be said to the opposite), but I'll leave it there, as a bit of food for thought.
Regardless of what it is, it's one of my favourite films and forms a huge part of the evolution of my likes with regards to films. I do think structurally, it has a lot more in common with Halloween and Friday 13th than it does with, say, Robocop, but sometimes, it's just a matter of perspective.