At once grimly determined and horribly thrilling — in a vicarious, sociopathic, first-person shooter video game sort of way — “Harsh Times” might be interpreted as a political statement about post-traumatic stress disorder and the law of diminishing returns for veterans of the Iraq war. But that’s really only a framework for the screenwriter and first-time director, David Ayer, to help explain the shattered and suicidally hell-bent psychology of Jim Davis, a jarhead given to joyrides that arouse and accelerate his predatory instinct.

Davis is a gung ho yet critically self-aware kind of monster. And since he’s played by Christian Bale, the live-wire flicker in his eyes and the blunt eloquence with which he articulates his actions have a cagey intelligence not usually associated with trained killing machines. Of course, these qualities are always part of Mr. Bale’s performances. His role here fits neatly into an accidental trilogy, linking to the hand-of-God avengers of “American Psycho” and “Batman Begins,” and more than suggests the actor is our current version of a 1970s Robert De Niro. He is so stitched into his character’s skin that it’s creepy.

At loose ends when he’s rejected for a job with the Los Angeles Police Department — good call, as viewers will learn — Mr. Bale’s near-broke and flashback-ridden Davis takes to aimless cruising with his similarly unemployed best friend Mike (Freddy Rodriguez of HBO’s “Six Feet Under”), whose live-in lawyer girlfriend (Eva Longoria of “Desperate Housewives,” true to type) insists he find a gig. Weak-willed Mike gives in to his buddy’s delinquent streak, and instead of going on job interviews he soon joins him in cruising the skankiest districts of L.A. In a few short hours, they hijack marijuana off dope dealers, beat and rob a crew of Mexicans unfortunate enough to cross their path, attempt to fence a stolen handgun, and generally get severely twisted. All in a day’s work. But this is only a prelude.

Mr. Ayer, who is best known for writing “Training Day,” uses a similar formula for this story. “Harsh Times” is very much a movie about being trapped in a car as a dangerous nut job slowly unravels into an even more dangerous nut job. Of course, when the nut job is your best friend, the process has a different vibe. And the actors dig into the roughhouse rhythms of homeboy vernacular and testosterone-driven idiocy that comes as second nature to old drinking buddies with too much time on their hands. The extensive use of exterior scenes, lent a sickly greenish glow, adds to the feeling of gritty, hyped-up reality.

Much as the maniac cop played by Denzel Washington in “Training Day”, though, Mr. Bale plays a single-minded terror with a secret soft side: He has a girlfriend in a sleepy Mexican village, whom he hopes to marry and bring to L.A. The contrast between the idyllic hillsides south of the border and the jagged realities of the city are supposed to imply that Davis is prey to forces beyond his control. As long as he is with his true love (played with sensitivity by Tammy Trull), he appears to be a strong, caring alpha male. But as the plot twists, Davis is invited to join a branch of federal drug enforcers, compelling a choice between the two sides of his personality. A fateful jaunt to Mexico ensues, and as his flashbacks worsen he begins to thoroughly flip out, taking his friends along for the death ride.

So much of “Harsh Times” hinges on Mr. Bale’s performance that the movie almost gets away with its intensely narrow focus. The problem is, without the actor to energize the dialogue and compel focus, the scenario is rather shamelessly akin to any number of other recent excursions through L.A.’s underbelly — it’s a cousin to “Collateral” and “Crash.” And unlike a really first-rate television series like “The Shield,” it never really expands to embrace an imaginative and complex moral dimension. The Iraq tie-in is convenient, but fails to make a convincing statement.

Certainly, nothing is as convincing as Mr. Bale’s murderous, gleeful glare and insatiable taste for casual violence. It’s a neat trick that the movie has you rooting for the guy, though in the end “Harsh Times” is an actor’s exercise that only pretends to be a social statement. Once the adrenaline subsides, you realize you’ve only been taken for the ride.