Morality is a funny thing. Most of us probably think we do a fine job separating Good from Evil. Murder is bad, helping the poor is good — intuitions like these form the backbone of our moral lives. One beautiful thing about gangster films, when done right, is their uncanny knack for cornering our commonsense morality into a grimy back alleyway way Uptown, dousing it in a gallon of lighter fluid, setting it ablaze and unloading a clip of bullets into its emaciated corpse. And as viewers, like doped-up addicts, we don’t notice a thing.

Now, replace commonsense morality with a helpless human victim, and put the gun in the hand of an Academy Award-winning actor. This is our introduction to Frank Lucas (Denzel Washington), legendary black gangster from New York’s bad old days of the ‘60s and ‘70s, for whom murder is as simple as profit maximization and pulling a trigger.

Ridley Scott’s epic, “American Gangster,” tries hard to sweep our commonsense morality off its feet to smuggle us into a moral Hell disguised as Heaven (not without the help of Denzel’s infectious screen presence). But this time, something just doesn’t sit right.

Unlike most mob movies, “American Gangster” gives us the story from both sides of the law. Stepping into the role of Essex County, N.J., police officer Richie Roberts is Russell Crowe, the film’s other Hollywood giant. Crowe is wonderful as the archetypal honest cop, an outcast among peers in an era of rampant police corruption (whose main representative is a vile Josh Brolin).

But in “American Gangster,” everything good comes twinned with the bad: though Roberts puts life and job on the line for the sake of justice, his domestic situation is a disaster. He can’t even find the time to raise his kid and prevent his wife (Carla Gugino) from shipping off to Vegas, with custody. No, he’s too busy playing ball with the guys and (shudder) romancing his divorce attorney for that. Standing in stark contrast, of course, is Lucas, whose filial piety and impeccable home furnishing skills make this street-tough gangster look saintly.

Murderous mama’s boy, meet the righteous womanizer. A clash of two personalities as different as the two banks of the Hudson from which each hails.

On this premise, the plot falls in place simply enough: Lucas rides his way to fame and glory on the trusty economic workhorse of Supply and Demand. He sells a premium product — pure, unmixed heroin branded “Blue Magic” — for half the price of the watered-down dope, effectively monopolizing the market, which had been humming along under the supervision of the Italians. Econ 1 also tells us that there ain’t no such thing as a free lunch, and Lucas ends up paying the price: he becomes Public Enemy Number One for venerable Detective Roberts.

As you can imagine, the twisting paths of Roberts and Lucas ultimately converge, leading to a provocative finish (based on truth stranger than fiction). All the while Scott fills the screen with stirring images of New York at its bleakest, the American Dream at its most elusive. Unfortunately, “American Gangster” just isn’t as complex as you want it to be. For every shot of Frank Lucas taking his mother to church, the film has one of a baby crying over a dead mother rotting away in the projects, another life surrendered to “Blue Magic.” This blatant counterpoint doesn’t create shades of moral gray, doesn’t blur lines of Good and Evil — it frustrates the viewer and makes Lucas all the more accountable.

“American Gangster” is a thrill to watch simply because Washington and Crowe are a thrill to watch. With Scott at the helm, bad old Harlem comes alive, and it drips with sweat, funk, blood, decay. The film has all the makings of a gangster classic but lacks the most important ingredient: a true moral gray area.