Everyone knows that when you have identical twins, one of them is going to be evil. That’s just science. But what happens when they’re BOTH monsters? That, my friends, is “Legend.”

Written and directed by Brian Helgeland (“42,” “A Knight’s Tale”), the film is about the Kray brothers, violent gangsters who were part of London’s swingin’ ’60s scene. How accurate the details are, I couldn’t say, though I note that the word “legend” appears in the title. Either way, it’s a bruising, entertaining piece of crime cinema, with Tom Hardy doing great work as both twins.

Reggie is the smart one, the personable businessman who can run a nightclub like a respectable member of society while administering brutal beatdowns in the back room. He has a nice girlfriend, Frances (Emily Browning), who serves as narrator and our introduction to the underworld (shades of “Goodfellas,” and not for the last time). When the American mafia, represented by Chazz Palminteri, visits to see about setting up a partnership with the Krays, Reggie and his shady businessman, Leslie Payne (David Thewlis), see it as a smart opportunity.

Reggie’s brother sees things differently. Ronnie is a psychopath, quite literally insane, and uninterested in playing nice to keep a low profile. He thinks being a gangster should be about busting heads, not making business deals. He’s also a homosexual, which is more problematic in the mid ’60s than it would be now, although being a violent criminal shields Ronnie from most persecution.

In addition to building his little empire of nightclubs and protection rackets, Reggie has to keep Ronnie under control, a full-time job in itself. Things come to a head midway through the film, when Reggie and Ronnie turn their fists against each other, ending in a crucial acknowledgement from Ronnie: maybe he’s not quite himself, sanity-wise. We need more of that — more of Reggie’s frustration over his problems being caused by his own twin, and more of Ronnie’s human side. The two are fascinating to watch, and their exploits are entertaining, but we never quite get inside their heads.

Our review of “Legend” from the Movie B.S. with Bayer and Snider podcast:

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Hardy does give two fully realized performances, though, playing Reg and Ron as entirely different characters. As Ronnie the psycho, he wears eyeglasses and (it appears to me) a plate of false lower teeth. His forehead doesn’t furrow expressively the way it usually does, as if Ronnie is too thick and savage for such delicate nuance. (I speak as one who has given serious contemplation to the details of Tom Hardy’s face.) As Reggie, Hardy is more recognizable, both in appearance and demeanor, and is no less imposing than Ronnie just because he’s not quite as violent. They carry themselves differently and have different speech patterns. In fact, there is enough difference between them (and the photographic trickery is so smooth) that you’re liable to forget it’s the work of a single actor.

“Legend” is ultimately too slight to enter the canon of great gangster films, and Brian Helgeland (as I’m sure he’d be the first to tell you) is no Martin Scorsese. But it’s a bloody, captivating mess as far as it goes, with career-best efforts from Tom Hardy. Next we’re hoping for a movie where he plays triplets.

B (2 hrs., 11 min.; R, pervasive harsh profanity, some graphic violence.)