Somber, fact-based crime drama “Black Mass” (Warner Bros.) chronicles the rise and fall of notorious Boston kingpin James “Whitey” Bulger.

Johnny Depp brings an intriguing blend of two-faced charm and menacing intensity to the central role. And the underlying values of this cautionary tale about the dangers of government collusion in wrongdoing are sound, as Bulger’s story is presented as a resounding admonition against using illicit means to achieve valid ends.

Yet, while the often harrowing bloodletting integral to the film is generally surrounded with an appropriate sense of dread, a note of exploitative excess creeps in as the story progresses. Thus even those few moviegoers for whom it can be considered tolerable may ultimately judge this gritty journey offensive.

Motivated by a misguided sense of ethnic and neighborhood loyalty, John Connelly, a childhood acquaintance of Bulger’s uses his position as an FBI agent to engineer a deal between the bureau and the initially small-time gangster.

According to the implicit terms of this corrupt bargain, Bulger is given free rein to expand his underworld empire in exchange for information about his rivals in the Italian-American mafia.

For Connolly’s boss,the goal seems clear: Use the small fry to catch the big fish. For Bulger, of course, it’s an obvious win-win: Use the feds to destroy your enemies, then take over their turf.

As Bulger’s uncontrollable barbarism causes events to spiral out of control, his rampaging ways threaten not only his G-men partners but his brother Billy (Benedict Cumberbatch), an ostensibly respectable and popular Bay State senator who waltzes around his sibling’s shady reputation.

Despite its indirectly religious title, and the unmistakably Catholic atmosphere in which its characters move, “Black Mass” devotes relatively little attention to faith.

We’re shown the fleeting image of a fatuous priest enjoying himself at a St. Patrick’s Day banquet where the Provisional IRA (to which Bulger has ties) is being praised. And Bulger sits silently in a church while brooding on his downfall. But otherwise religious expression is confined to the scenery.

So it’s not irreverence but rampant indifference to human life that may give even the heartiest ticket buyers pause.

The film contains frequent brutal violence with considerable gore, mature themes, including prostitution, about a dozen uses of profanity and pervasive rough and crude language. (L, R).