The Momart fire in 2004 destroyed scores of important artworks. Tracey Emin’s notorious tent, in which she had embroidered the names of everyone she had ever slept with, was burnt. Pieces by Helen Chadwick, Rachel Whiteread and Damien Hirst were torched. But the really critical loss was the huge installation by the Chapman brothers, called Hell, that had been the centrepiece of the Royal Academy’s Apocalypse show. Hell was a great evocation of human evil, acted out by vast numbers of toy soldiers on a relentless scale. It had taken years to make. Those of us who were lucky enough to have seen it stored the sight away in our memories and assumed that was that.

The Chapmans are not like other artists, however. They don’t get deterred. They don’t get maudlin. They finish the Airfix kit. And this hard-core determination of theirs led them to announce that they would be remaking Hell, and that this time it would be bigger. The results have now been unveiled at the White Cube gallery in St James’s. They are nothing less than astounding. Hell, in its new, extra-large form, is so ambitious, so ghastly, so sick and so brilliant that I hereby nominate it as the key contemporary artwork of our times.

Nine large glass cases arranged on the floor in the shape of a swastika have once again been filled with zillions of scenes of murder and mayhem, acted out by countless numbers of Nazi soldiers and their mutilated victims. As in the original Hell, the central case contains an erupting volcano that is spewing evil to the four arms of the swastika. One journey ends on a ruined church, in which Hitler is being baptised. Another ends on a ruined pseudo-Greek temple that turns out to be a McDonald’s franchise. A third ends on a factory of death; the fourth on a killing field piled high with corpses.

Other artists have chosen to memorialise human wickedness by resorting to evocative abstractions – the Washing-ton memorial to the soldiers lost in Vietnam being the ultimate example. But the Chapmans seem to view abstraction as a get-out clause. Hell is Auschwitz reenacted in full, gory detail. Goya favoured the same approach. Don’t avoid it. Show it. On and on it goes. Carnage and dismemberment in every direction. Once the impact of the whole of Hell has bowled you over – which it will – the thing to do here is to wander among the cases picking out the absorbing close-ups. Spot the little attic with the toys in it belonging to Anne Frank. Note the calm figure of Hitler standing by his easel and enjoying his hobby. The deranged detailing is hilarious, in that dark and outrageous Chapman manner.

Don’t let the sick humour fool you, though. In its heart, this immense piece of creative nihilism isn’t only attacking Hitler and the Nazis. Hell is a scale model of a cycle of evil that shows no sign of ending. It’s having a go at all of us.

Elsewhere in the gallery, further Chapman mischief is afoot. To complement Hell, the brothers have also acquired a set of real watercolours by Hitler, which they have added to and redrawn. The fact that Hitler was a keen amateur artist is widely known. What isn’t perhaps as clear is that he was a really bad one. His friends would receive twee postcards, painted with little landscapes and flowerpots, which he would sign “A Hitler” in the childish handwriting of a 15-year-old schoolboy.

The Chapmans have bought a stack of these and covered them with childish squiggles of their own, of rainbows, flower patterns and butterflies. It’s a mock language of innocence, created deliberately, you feel, to match the mock innocence of the 20th century’s worst monster. The idea that the man responsible for the genocide of the Nazis should have painted watercolours of roses is, indeed, as obscene as it is telling. As Hannah Arendt pointed out so astutely, evil is banal. It’s everywhere. Under all our noses. And the banality of evil is the Chapmans’ greatest theme.

To complete their fiendish investigation of the rest of us, the brothers have also acquired a set of family portraits, mostly Victorian, which they have also defaced by adding misshapen mouths and cancerous eyes to the poor, unsuspecting dads and grandmas. We usually have our portraits painted to commemorate our lives. The Chapmans seem keener to commemorate our disintegration. It’s another wicked and hilarious wheeze.

I advise you to get over here straightaway. This is modern art at its most brilliant and most pertinent.

Jake and Dinos Chapman: If Hitler Had Been a Hippy How Happy Would We Be, White Cube, SW1, until July 12

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