In 2000, two former Boston Globe journalists published an investigative account so fantastic it could only have been true. In the 1970s and ’80s, the Federal Bureau of Investigation was so fixated on taking down the Italian mafia in Boston that it allowed a notorious South Boston criminal who was its informant to build his own empire. James “Whitey” Bulger fed scraps to the FBI starting from 1975, and over the years, he used the agency to wipe out his rivals and expand his criminal enterprise. Two FBI agents, John Connolly and John Morris, were so enamoured of Bulger – Connolly especially since he shared the gangster’s South Boston background – that they fudged reports, ignored Bulger’s spree of murders, leaked information back to the ganglord, and eventually became his partners on the inside.

Written by Dick Lehr and Gerard O’Neill, the meticulously researched and riveting Black Mass: Whitey Bulger, the FBI, and a Devil’s Deal inspired elements of Martin Scorsese’s The Departed, the 2006 crime drama about a police mole and an undercover operative. The Departed’s flamboyant villain Frank Costello was said to have a tribute to Bulger, but while Scorsese played Costello’s crimes for grim laughs, Scott Cooper’s adaptation never loses sight of Bulger’s bottomless brutality.

From bumping off adversaries and troublesome gang members to manipulating the FBI agents into doing his will, Johnny Depp’s Bulger is the Devil with a touch of the vampire. Depp’s perfectly chiseled features and charmingly idiosyncratic roles routinely place him at the top of “Sexiest Man Alive” lists. Depp’s rivetting image scrub involves contact lenses, a hair-piece and prosthetic facial make-up that gives him a sickly pallor. Nowhere is the actor’s resemblance to the anti-Christ more evident than in the scene in which he stands apart from the rest of the church congregation at his mother’s funeral service.

Chilling but also cold

Cooper’s movie is evenly placed, smartly written and beautifully performed, and it benefits tremendously from Masanobu Takanayagi’s elegant and richly atmospheric camerawork. But Black Mass cannot escape the perils of reducing a complicated maze of deep deception, corruption and ineptitude into a workable running length. Over 124 minutes, the screenplay by Mark Mallouk and Jez Butterworth zips through Bulger’s recruitment as a “top echelon informant”, his manipulation of his access to the FBI to slaughter his rivals, and his relationship with his Senator brother Billy (Benedict Cumberbatch). The whole thing plays out against a backdrop of tribal passions, fierce loyalty to one’s own kind, and a spirit of clannishness that allows an old woman whom Bulger is gallantly helping out to cheerfully ask him when he got out of jail.

As the real villain of the piece, the weasly Connolly (Joel Edgerton), who subordinates his employer’s interests to favour a thug from the neighbourhood, might have made a fine contrast to Bulger’s calm mayhem. Edgerton wears his deception well, but his character jostles for attention in a big-name cast that includes Kevin Bacon as Connolly’s skeptical boss, Rory Cochrane as Bulger’s associate Steve Flemmi, Corey Stoll as the prosecutor who takes down Bulger, and David Harbour as FBI agent John Morris, who quietly backs Connolly’s rule-breaking behaviour because he too has been seduced by Bulger.

Morris’s tragedy was a central pillar of the Boston Globe reporters’ book, but in the movie, he is only one of several witnesses to Bulger’s horrific slayings (this is the kind of movie in which a bullet in the head is the kindest way to go).

The killings are swift, brutal and plentiful, executed with a businesslike chill that hangs over the whole movie. Black Mass is a better adaptation of non-fiction books than most, but the experience is ultimately as cold as Bulger’s heart.