The Brothers Grimm are back. Jake and Dinos Chapman — purveyors of pornographic grotesqueries — today open their latest show. The visitor might be forgiven for thinking that he had got lost and had strayed into the Royal Academy's Summer Show. What are all those timid little watercolours doing on the walls?

Predictably, there's a sinister twist. The diligent little architectural studies, those insipid landscapes and amateurish sketches are the (apparently authenticated) works of Adolf Hitler. The Chapmans have adorned them with their own additions: butterflies and shooting stars and jolly rainbows.

The show's title, If Hitler Had Been a Hippy How Happy Would We Be, doesn't have a question mark. But it drops a pretty disconcerting one into our heads. For all that we may want to look at the bright side, this show disturbs more discomfiting possibilities.

The Chapmans have challenged us in this way before. They notoriously defaced a set of etchings by Goya. But every work of art is caught up in the momentum of progress, the Chapman brothers suggest. Each new piece is made at the cost of a past that is lost. Destruction can be a creative act.

There is nothing particularly novel about this. In fact, it gave Modernism its impetus as Rauschenberg famously illustrated when he erased a drawing by his fêted predecessor, De Kooning. The Chapmans find a dramatically audacious way of making the point by imposing their own handiwork on to pieces by someone who probably counts as the most famous person of the 20th century.

As these adapted watercolours find their context in a wider body of work the implications grow more complex. In the next gallery space is F*****g Hell — an even more elaborately appalling version of Hell: an apocalyptic tableau of Nazi figurines engaged in an orgiastic frenzy of violence that was destroyed in a warehouse fire.

If Hitler had not been turned down by the Viennese Academy would we never have had the Holocaust? Would we never have seen the sort of appalling Nazi spectacles that now squirm before our stare? It's a question many must have asked. But the more uncomfortable one to answer is: do we need this Hell? Does the human psyche crave its horrors? Can we contemplate the world without it? Or, if it was lost, would we — just as the Chapman brothers have done — re-create it?

In a third gallery hang rows of disfigured oil portraits. Long lizard tongues, leprous excrescences and peeling flesh mutilate the faces. Strangely, they feel almost traditional. Do we contemplate them as all those lost medieval saints once contemplated the skull beneath the skin? Are gruesome physical threats the flipside of a higher spiritual life? Maybe we cannot be happy without Hell, and maybe we need artists as baffling, enraging and even potentially immoral as the Chapmans.

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