After the years of glory, Gilles seemed to have found life unbearably dull. During the course of the following year, according to his later confession, he committed his first sex murder, that of a boy. His grandfather willed his sword and cuirass to the younger brother René but died in the following year. Gilles was suddenly able to do what he liked.

One day, a young boy dubbed Poitou was brought to the château and raped, after which Gilles prepared to cut his throat. At this point, Gilles de Sille pointed out that Poitou was such a handsome boy that he would make an admirable page. So Poitou was allowed to live, and become one of Gilles' most trusted mignons.

Gilles' attacks of sadism seem to have descended on him like an epileptic fit, and turned him into a kind of maniac. A boy would be lured to the castle on some pretext, and once inside Gilles' chamber, was hung from the ceiling on a rope or chain. But before he had lost consciousness, he was taken down and reassured that Gilles meant him no harm. Then he would be stripped and raped, after which Gilles, or one of his cronies would cut this throat or decapitate him (they had a special sword called a braquemard for removing the head).

But Gilles was still not sated; he would continue to sexually abuse the dead body, playing with the head in grotesque manner, sometimes cutting open the stomach, then squatting in the entrails and masturbating. When he reached a climax he would collapse in a faint, and be carried off to his bed, where he would remain unconscious for hours.

His accomplices would meanwhile dismember and burn the body. On some occasions, he later confessed, two children were procured, and each obliged to watch the other being raped and tortured.

Gilles was not merely sexually deranged; he was also a reckless spendthrift. He surrounded himself with a retinue of two hundred knights, for whom he provided. He loved to give banquets and fêtes; in 1435, when the city of Orléans celebrated its deliverance by Joan of Arc, Gilles presented a long mystery play about the siege, with enormous sets and a cast of hundreds, playing, of course, the leading role himself. He also provided food and wine for the spectators. Like a Roman emperor he must have felt that he was virtually a god.

In a mere three years he had spent what would now be the equivalent of millions of dollars. Back at Machécoul, he had to sell some of his most valuable estates. His brother was so alarmed that he persuaded the king to issue an interdict forbidding any further sales of land. For a man of Gilles' unbridled temperament, this was an intolerable position. He went into a gloomy and self-pitying retirement.

Years before, when he first went to court, he had borrowed a book on alchemy from an Angevin knight who had been imprisoned for heresy. Alchemy was prohibited by law, and for a man with Gilles' romantic craving for "the forbidden," this must have been an additional incentive to learn more about it. Now, ten years later, with his coffers empty, he realised that black magic might be the answer to his problems.