Let me tell you about G. In his teens, he met a young woman he adored, and they had three children. To support the family, he worked as a builder’s sidekick. The builder introduced him to amphetamines to help him work long hours and bring in more money, but it wasn’t long before he was spending that extra money on more amphetamines, and then…

Fast forward. G’s problems with speed are such that he is declared mentally ill. Which is true enough: phets fuck your head up. But the label ‘mentally ill’ gives G a label to hide behind when he’s dealing with the authorities. And he’s doing a lot of that now, his relationship having fallen apart.

For a while, G is put in a ward with men in their seventies and beyond. He steals from them, offers his body to them in return for money that he spends on drugs. And moves from there to a hostel where he is ultimately expelled for dealing drugs to other residents. By this time he’s using crack and heroin as well. It’s possible that by introducing another hostel resident to amphetamines that he initiated an incident in which that resident attacked a staff member with a knife. That person’s life was saved principally because a colleague also on duty had serious martial art training.

I mention G — a real person — because he is unquestionably the closest I have come to meeting a worthless human being. But does such a vile individual deserve to be killed because of his socially corrosive behaviours? That’s the question which underpins Harry Brown, a British film of sombre power which I urge you to see as soon as you can.

Michael Caine plays the title character, a pensioner and former marine who loses his wife and best friend in rapid succession, the latter to the gang of feral youths who rule the rundown estate the action plays out on. Writer Gary Young’s script is stark and apparently simple, each piece placed precisely to create a morally complex story that’s a world away from most vigilante films. Daniel Barber’s direction is terse, a sober palette and well-judged music bringing out the atmosphere and nuances of Young’s writing. And Caine’s performance is subtly powerful, proving once again that given the right material the man really can act.

For the most part, the story is credible in its action. Arguably the climax is overegged, but to me the weight of the emotions and ideas justified a move into high stakes territory that’s a touch Hollywood in comparison to what’s come before. It’s powerful stuff, make no mistake, and bridges lo fi realism with a more mythic dimension — the subway tunnel that the gang hangs out in is a potent symbol of the hell that the estate has become, and if you’re thinking that the dealers Michael Caine despatches at the start of the film are caricatured I can assure you they’re credible to anyone who’s met the likes of G and his associates.

Harry Brown is a potent social drama that would be easy to mistake for an action film, and some may look on it in that way. It’s evenhanded in its portrayal of the people who Harry targets: it’s clear that they’re victims not just of a gun-toting Caine but a fucked-up welfare system and the abandoned families in its charge. Equally though, they’re morally bankrupt thugs perpetuating a culture of violent drug-related crime. Put those two pictures together, and Michael Caine in the middle of them packing a .38, and it’s clear why this is a rich film that will inspire debate.

As for G…I wouldn’t kill him. Not myself. But I can understand why others would. If he does come to a sticky end, it will likely not be at the hand of a vigilante, but of someone in the criminal culture he’s part of and ineptly tried to rip off. Will I mourn him? I doubt it. But I’d spare a thought for his former partner and their children, victims of a weak and callous user who loved amphetamines more than he loved himself.