Well it's no coincidence that American mainstream horror films are much darker and much much more violent than they've been in a long time. It's an indirect result of, maybe not terrorism, but the tone of the world since 9/11 has happened.
I can't say American mainstream horror has become darker because of 9/11...if anything it's been nothing but remakes & prequel/sequels for the past 10 years.
As for being more violent, slasher films have been passing themselves off as horror (replacing suspense) for a long time now.
B-grade horror is nothing but senseless killing.
Maybe the number of zombie films since 9/11 have an underlining theme of bio chemicals wiping out humans in a grand scale & surviving to fight back.
I think if you're trying to put a connection between a 9/11 undertone to movies, then it would be more reminiscent to several sci fi flicks like Battle Los Angeles, Skyline & SourceCode.
Well, yeah, I agree with you. I think horror or scifi with a more apocalyptic edge is more in tune with the 9/11 influence.
Though I can think of a line from Dennis Hopper in Land of the Dead where he says; "I don't negotiate with terrorists."
In the film The Mist, the scene where the opaque white mist shrouds the supermarket, while people run for their lives is also reminiscent of the powered clouds that draped New York, though I don't know if this was intentional.
That's a cliched quote that was also used in the 1st Die Hard & Harrison Ford as the president in AirForce One...pre 9/11.
The thing is, I don't mean to sound cold or insensitive but I'm not American nor do I live in America so, "the tone of the world since 9/11" (BackFire's quote) has very little reasoning or context to everyday life & views since 9/11.
"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."