Are you sure Boyle said "horror" and not "zombie"? The latter would make more sense since 28 Days Later is a horror movie. Weather it's a zombie movie is more debatable.
It could have been zombie, in fact I think it might have been. I remember thinking it was funny because Alex Garland wrote it as an unashamed homage to Romero.
Interesting, the connections we make. Technically speaking,
28 Days Later isn't a zombie movie at all (since the "creatures" in it aren't dead, just diseased), and yet--because they feed on human flesh--everyone called it a zombie film. Guess if it looks like one and acts like one. . . .
I disagree (not about posting more, you should), Alien is a horror film. You can wrap it in SF trappings all you like, it's still a haunted house movie.
Horror films attempt to be scary by bending toward gratuitous, graphic violence.
That's usually a generalisation normally used by people who are derogatory towards the genre of horror as a whole. But it's a broad statement. The Texas Chainsaw Massacre (which was a HUGE influence on Ridley Scott and Alien) has no graphic violence. The Exorcist has very little. The Shining. The Omen. Halloween. None of these movies have any more graphic violence than Alien. Yet you'd be hard pressed to call them anything but horror movies.
I think probably what you're thinking more of are things like Saw, which I agree are generally a waste of time. But you infer a lack of intelligence in horror films, which again is an oversimplification, like saying all fantasy films are for adolescents and people who refuse to grow up. For example, Dawn of the Dead contains much graphic horror and wears its horror film tag on its sleeve, but it's known as much for its intelligence and satirical and social qualities as it is for its gore. David Cronenberg's films are fiercely intelligent.
Besides, Aliens is an action film 
Much as I'd like to, I can't really argue with you here. I don't infer a lack of intelligence in horror films as a whole (we can all think of some brilliant ones); but I'll cop to the generalization bit, and even to being a bit derogatory toward horror films. You make some good points here.
Still, I'm not yet ready concede all. I still think
Alien is far more a thriller than a horror film. But, hey, maybe that's just me. . . !
Please post more!
Thanks for the sentiment. I wish I could spend a lot more time here than I do. That's been a problem for years now. . . .
- Uni