God Bless America, Bobcat Goldthwait, 104 mins (15)

A middle-aged man and a crazed teen wade through the shallows of modern America

Tara Lynne Barr is mesmerising as Roxy in this cheerfully vicious satire

American comedian turned film-maker Bobcat Goldthwait has contributed more than his share to what Martin Amis would call the "moronic inferno" of modern culture. A regular guest on US talk shows, Goldthwait made his mark with a wise-ass persona and a high-pitched, dental-drill delivery. He's also appeared in three Police Academy films – a crime which has its own paragraph in the Geneva Convention.

Now it looks as if he's paying penance in his fifth feature as director, God Bless America. This cheerfully vicious satire addresses the horror of American culture today, through a hero who's had enough of it all – enough of demagogue TV hosts, placard-wielding bigots, no-talent "talent" shows, and neighbours who show you total contempt while greeting you as "bro". The film's comprehensive rage recalls Marlon Brando's Johnny in The Wild One who, asked what he's rebelling against, replies, "Whadda ya got?" Even poor Woody Allen is not exempted from the film's condemnation, although – for reasons I haven't quite fathomed – Alice Cooper is.

Frank (Joel Murray) is, to quote Taxi Driver, the quintessential "man who would not take it any more". Tubby, divorced Frank spends his days enduring the racket of his idiot neighbours – their frenetic TV fodder, their bickering about the crucial current affairs of the day: "Oh my God what is wrong with Lindsay Lohan?" What particularly gets Frank's goat is their perpetually crying baby: for Goldthwait, contemporary America is a nation of spoilt brats of all ages wailing their heads off. All this is driving Frank to homicide – "I know it's not normal to want to kill," he muses lucidly, "I also know that I am no longer normal" – and before long, he makes his grievance known in a way that's bloody, outrageous and taboo-flouting. OK, it's only an imagined moment of wish-fulfilment, but you'll laugh, once you've eased your jaw back into place.

Frank's sorrows mount up. He has to deal with his contemptuous ex-wife and horrible daughter; he has to listen to his co-workers cackling derisively about the latest lamentable "freak" offered as bait on a talent show called American Superstarz; he is informed by his doctor (a priceless scene with Dan Spencer) that he has a brain tumour; and he loses his job, essentially for being considerate.

This modern martyr finally decides to start killing, for real this time, and his extreme measures win him the adoration of a cheerfully demented teenager named Roxy (mesmerisingly abrasive newcomer Tara Lynne Barr). Roxy actually outdoes Frank's loathing for the modern world: while he's tolerant of certain phenomena, she hates pretty much everything, except Alice Cooper.

This odd couple team up for a killing spree, although Frank cools down Roxy's eager Bonnie and Clyde fantasies by insisting they stay platonic: "I refuse to objectify a child. I'm not American Apparel." Their subsequent adventures gratify those revenge dreams that we've all entertained. In one scene, the duo slaughter some teenagers for giggling through a documentary about the My Lai massacre. The punchline comes when the incident is covered on the TV news, and the violent content of the documentary itself is blamed for the killing. No one, Frank observes, takes responsibility for their actions any more – although he and Roxy do, and that's what makes them sympathetic.

You may recognise the jowly Joel Murray as the unfortunate Fred Rumsen in Mad Men, and he's brilliant casting as Frank, with his laconically pained calm. Most importantly, he makes Frank come across as a deeply nice man – perhaps, Goldthwait's saying, the last nice man in America. For what Frank laments isn't just noise or vulgarity but the death of compassion, tact, kindness and (a deeply unfashionable word) shame. He hates American Superstarz not because it's dumb but because it's modern bear-baiting, encouraging America to revel in the degradation of the weak and deluded. "Why have a civilisation," Frank asks, "if we're not interested in being civilised?"

Frank's odyssey climaxes in a confrontation with TV culture that recalls Peter Finch's "I'm mad as hell!" outburst in Network. Throughout the film, Frank and Roxy's jeremiads make you aware how much Goldthwait is putting his scripted thoughts into their mouths (sometimes decanting whole chunks of his own stage routine). But they are eloquent thoughts, and patently sane. There's political fury here, directed at America's hectoring shut-your-mouth Right – but more generally, a plea for being properly adult.

This is a rare and refreshing thing, an American film about despair – and funny with it. You'll chuckle darkly rather than laugh out loud. (Now there's a phrase that has been degraded.) But the film's final rant at the world is a wonderful lucid epiphany, and that's where, through Frank, Goldthwait transcends his TV doofus persona to emerge as a trenchant truthsayer – Lenny Bruce by proxy.