Michael Winterbottom's cinematic adaptation of Jim Thompson's novel is a failure because it's too exact; it's too precise and pristine, and too close to the source material.
These issues have and can work for other directors: think Kubrick's overly sanitized and creepy 2001: a Space Odyssey (1968), or John Huston's faithful The Maltese Falcon (1941), which takes most of its dialogue straight from Hammett's book. Keep in mind, however, that these tones fitted with the material given, which is not what I can say about Winterbottom's film.
Thompson's classic pulp story is narrated by its protagonist, a psychopathic homicidal sheriff whose good ol' boy demeanor hides a calculating and mentally unhinged persona. He is a fascinating and frightening character, because we, the reader, are allowed access into his head. But he is telling the story to us, and since he gets less and less control over his impulses as the book goes on, his status as a reliable narrator becomes more shaky until it explodes in our face.
But if there is something that Winterbottom film is, it's reliable. What's presented to us is a accurately designed 1950's Texas town, the characters and most of the events are taken straight from the pen of Thompson. The awful, gross, and explicit bursts of violence are presented in a straightforward manner. Heck, the only real deviation is the sex scenes, which are shown in great detail, instead of the novel's suggested obviousness.
And that's the problem: it's too obvious! Part of what makes the book of The Killer Inside Me so brilliant is how shaky, flakey, and willing the story is to pull the rug out from under your feet. It's comparable to being on a tightrope, and suddenly finding that the net has vanished. Much of the film is shot in close-ups, but in such a way that it reveals only the external, and not the internal. (except for the last shot, which is just plain pretentious). Though it gives itself the air of a deep film about a psycho, it ends up a distant and surprisingly dull piece of celluloid (or digital...whatever format it was shot in).
One more thing: I first heard of The Killer Inside Me the book, from hearing about the hype from the film before it premired. I thought the story sounded interesting, and finding out that this sordid subject was based on a well-regarded pulp novel from 1950 gave me the inciative to find and read that book. And now it's one of my favorite novels. The film, when I finally got around to seeing it, is a simply a big disappointment. If the story still interests you, do yourself a favor: read the book, skip the film.