James “Whitey” Bulger is just a thug, but in the personification by Johnny Depp, he becomes a seething, sneering, smoldering volcano, spewing out venom at everyone around him, and eventually destroying everything in his path with his molten-lava pathology, and explosive anger.
Whitey grew up on the South Side of Boston, where streets were rough and allegiances were forged deeply. Whitey's brother, Billy (Benedict Cumberbatch) became a State Senator, and later, Chancellor of the University of Massachusetts. Whitey's childhood chum, John Connolly (Joel Edgerton) became an FBI agent stationed in Boston, but actually, that's where the real trouble began.
Whitey might have been consigned to just low-level street crime, a little numbers game, some minor drug dealing, but he quickly runs up against the (Italian) Mafia, which didn't take kindly to back-alley Irish guys infringing on their territory. Something had to give. So Whitey makes an unholy alliance with the FBI, through his boyhood friend John, to become an informant for them in exchange for information leading to the pursuit of the Mafia. Everybody wins, right? The FBI gets to brag that it beat the Boston Mafia, John Connolly gets plaudits and promotions, and Whitey has some elbow room to grow his little operation, while letting the Feds take care of his rivals.
Ah, but in that vacuum steps the greedy monster that “Whitey” Bulger has become. Once he loses his young son to a sudden illness, he doesn't really have any affection for anyone any more. His wife realized she meant nothing to him, and left. So did John's wife, after she grew increasingly uncomfortable with the company John continued to keep. That one scene where Bolger, by feigning concern over her fake illness, manages instead to establish his mastery over her with a fawning creepiness, is a scene even more chilling than any of the violent “hits.” But it's entirely consistent with Bulger's character: he was the sun around which everything in his universe revolved, pulled into the vortex of his gravitational field; doomed to extinction if his favor no longer shined on them.
Johnny Depp is hardly recognizable from the Jack Sparrow character he flaunts in the “Pirates of the Caribbean” series. Though his character is not made to look physically attractive, he's still impossible to ignore. He's a force of nature. And he manages to blacken with soot and ash all who are foolish enough to draw near, because anyone close to him becomes a co-conspirator. Or they die.
When a new prosecutor rolled into town and the vultures started to circle, “Whitey” Bulger eluded them all, spending 12 years on FBI's Most Wanted List, until they finally caught him in California from an anonymous tip. He was convicted for 11 murders, but that's only the ones they could prove. And everyone around him fell from grace as he did, a true paragon of unrepentant evil.