The art of Adolf Hitler (with a little help from the Chapman brothers)

When the artists Jake and Dinos Chapman bought a series of paintings by Adolf Hitler for £115,000, many questioned the morality of paying for works produced by one of history's most brutal dictators.

Yesterday, the brothers unveiled 13 of the watercolours, on which they had added psychedelic rainbows, stars and love hearts, and placed them back on the market for £685,000.

The Chapmans denied that the paintings, which are selling as a single work, sought to "redeem" Hitler, and classed the original watercolours as "bland" and lacking in talent. "The idea of redeeming Hitler is bad, the idea of redeeming his work is a staggering work of genius," said Jake.

He hoped the defacement of Hitler's work, which includes landscapes, vistas of Roman ruins and still life, which the dictator painted when he was young, would have him "spinning". The changes they had added meant it was no longer Hitler's work, he added.

"If hell exists and Hitler exists in it, he would be spinning if he saw these. It's not his work any more. It's our work," he said.

Following concerns that the work could be bought by Nazi sympathisers, White Cube Gallery in London, where it is showing, has stated that it will be extremely careful about who it sells to.

Dinos Chapman said the work, entitled If Hitler had been a Hippy How Happy Would We Be, was a rumination of what might have been had Hitler not been refused entry to Vienna's art school. He added they showed a "blankness" rather than any hint of the deadly pathology that he would later demonstrate.

"He tried to get into art school with these. They are bland and show no presentiment of the genocide to come. They represent the husk of a man who would be filled up with bitterness and hatred. They are identical to thousands of drawings in junk shops. All they demonstrate is that they are a terrible work of art, not that the person behind them will become a tyrant," he said.

This is not the first time the brothers have defaced artwork. They offended some Spanish critics when they reworked 80 etchings by Goya, Disasters of War, adding funny faces and clowns heads.

The Chapmans also unveiled a swastika-shaped sculpture of epic scale, based on Hell, a work destroyed in the Momart fire in east London in 2004 which was bought by Charles Saatchi for a reported £500,000. This new version, Fucking Hell, sold for £7.5m.

The critically-acclaimed original installation depicted thousands of miniature Nazi soldiers carrying out acts of mass torture and cadavers hanging off trees. It was the centrepiece of the Royal Academy's Apocalypse exhibition in 2000. When the fire destroyed the work, the brothers announ-ced they would recreate it. "As an event, we couldn't fail to see something funny about hell being on fire. We couldn't imagine a world without hell and we wanted to rescue the work from the sentimentality that some clothed it in after it was burned. There was an affection for the work that did not exist when it was there as an object before the fire," said Jake Chapman.

Eighteenth and nineteenth century-style aristocratic portraits which the Chapmans doctored to incorporate ghoulish masks, and deformations, entitled One Day You Will No Longer Be Loved, is also on show.