When his father died in January 1767, Sade assumed the title "Comte de Sade", although he never used this title and continued to refer to himself as the "Marquis de Sade".

Scandals and imprisonment
Shortly after his wedding, he began living a scandalous libertine existence and repeatedly abused young prostitutes and employees of both sexes in his castle in Lacoste, Vaucluse, later also with the help of his wife. His wayward behavior also included an affair with his wife's sister, who had come to live at the castle.

After an episode in Marseille that involved poisoning of prostitutes with the supposed aphrodisiac spanish fly (nobody died), he was sentenced to death for sodomy and poisoning in 1772 but was able to flee to Italy and later hid at Lacoste. His mother-in-law obtained a lettre de cachet for his arrest. He kept a group of young employees at Lacoste, most of whom fled quickly. The father of one of these employees came to Lacoste to claim her, shot at the Marquis and barely missed.

In 1777 Sade was finally arrested and imprisoned in the dungeon of Vincennes. He successfully appealed his death sentence in 1778, but remained imprisoned under the lettre de cachet. He escaped but was recaptured soon after. In prison, he started to write. At Vincennes he met the fellow prisoner Comte de Mirabeau who also wrote erotic works, but the two disliked each other immensely.

In 1784, Vincennes was closed and Sade was transferred to the Bastille in Paris. On July 2, 1789, he reportedly shouted out of his cell to the crowd outside, "They are killing the prisoners here!", causing somewhat of a riot. Two days later, he was transferred to the insane asylum at Charenton near Paris. (The storming of the Bastille, marking the beginning of the French Revolution, happened on July 14.) He had been working on his magnum opus, "The 120 Days of Sodom" and he despaired when the manuscript was lost during his transferral; but he continued to write.

He was released from Charenton in 1790, after the new Constituent Assembly had abolished the instrument of lettre de cachet. His wife obtained a divorce soon after.

Return to freedom, and imprisoned for "moderatism"
During his time of freedom (beginning 1790), he published several of his books anonymously. He met Marie-Constance Quesnet, a former actress and mother of a six year old son who had been abandoned by her husband; Constance and Sade would stay together for the rest of his life. Sade was by now extremely obese.

He initially arranged himself with the new political situation after the revolution, called himself "Citizen Sade", and managed to obtain several official positions despite his aristocratic background. He wrote several political pamphlets. Sitting in court, when the family of his former wife came before him, he treated them favorably, even though they had schemed to have him imprisoned years earlier. He was even elected to the National Convention, where he represented the far left.

Appalled by the Reign of Terror in 1793, he nevertheless wrote an admiring eulogy for Jean-Paul Marat to secure his position. Then he resigned his posts, was accused of "moderatism" and imprisoned for over a year. He barely escaped the guillotine (probably due to an administrative error) and was released after the overthrow and execution of Robespierre had effectively ended the Reign of Terror. This experience presumably confirmed his life-long detestation of state tyranny and especially of the death penalty.

Now all but destitute, in 1796 he had to sell his castle in Lacoste that had been sacked in 1792. (The ruins were acquired in the 1990s by fashion designer Pierre Cardin who now holds regular theatre festivals there.)

Imprisoned for his writings, return to Charenton, and death
In 1801, Napoleon Bonaparte ordered the arrest of the anonymous author of Justine and Juliette. Sade was arrested at his publisher's office and imprisoned without trial, first in the Sainte-Pélagie prison and then, following allegations that he had tried to seduce young fellow prisoners there, in the harsh fortress of Bicetre. After intervention by his family, he was declared insane in 1803 and transferred once more to the asylum at Charenton; his ex-wife and children had agreed to pay for his pension there.

Constance was allowed to live with him at Charenton. The liberal director of the institution, Abbe de Coulmier, allowed and encouraged him to stage several of his plays with the inmates as actors, to be viewed by the Parisian public. Coulmier's novel approaches to psychotherapy attracted much opposition.

Sade began an affair with thirteen-year-old Madeleine Leclerc, an employee at Charenton. This affair lasted some 4 years, until Sade's death in 1814. He had left instructions in his will to be cremated and his ashes scattered, but instead he was uried in Charenton; his skull was later removed from the grave for phrenological examination. His son had all his remaining unpublished manuscripts burned; this included the immense multi-volume work Les Journées de Florbelle.

Quotes
"Imperious, choleric, irascible, extreme in everything, with a dissolute imagination the like of which has never been seen, atheistic to the point of fanaticism, there you have me in a nutshell.... Kill me again or take me as I am, for I shall not change."

— Marquis de Sade, Last Will and Testament

Literary works
Many of Sade's works contain explicit and often repetitive descriptions of rape and countless sexual perversions, often involving violence and transcending the boundaries of the possible. Sade's libertines founded their philosophy on a purposeful flouting of moral norms and a hatred of religious ethics. In nature, they say, the strong win and the weak lose; therefore all laws and ethics, designed as they are to protect the weak, are seen as unnatural.

In 1782, while in prison, he completed the short Dialogue Between a Priest and a Dying Man, expressing his atheism by having the dying libertine convince the priest of the mistakes of a pious life.

The novel The 120 Days of Sodom, written in 1785 but not completed, catalogs a wide variety of sexual perversions performed on a group of enslaved teenagers and is Sade's most graphic work. The manuscript was believed to have been lost during the storming of the Bastille and the book was not published until 1904.

In 1787 he wrote Les infortunes de la vertu, an early version of Justine which was published in 1791. It describes the misfortunes of a girl who continues to believe in the goodness of God despite persistent evidence to the contrary. The companion novel Juliette (1798) narrates the adventures of Justine's sister, Juliette, who chooses to reject the teachings of the church and adopt an amoral hedonist philosophy, resulting in a successful fulfilled life.

The novel Philosophy in the Bedroom (1795) accounts the lascivious education of a privileged young lady at the dawn of womanhood. The work is structured as a play and is concise, witty and engaging; the archetypal Sadean characters are, here, used most effectively. The book contains a lengthy political pamphlet Frenchmen! One More Effort If You Wish To Be Republicans! in which Sade advocates for a utopian form of socialism. He states that laws against theft are absurd: they protect the original thieves, the wealthy, against the poor who have no option left but theft. He also argues that the state has no right to outlaw murder if it continues to sanction institutionalized murder in the form of executions and war. Laws against blasphemy are seen as pointless: they are not needed if God doesn't exist, and if He does, he surely won't be petty enough to care about minor attacks. The pamphlet was reprinted separately for distribution during the revolution of 1848.

In Aline and Valcour (1795) he contrasts a brutal African kingdom with a utopian island paradise. This was the first book published under his true name.

In 1800 he published a four-volume collection of short stories titled Crimes of Love. In the introduction, Reflections on the novel, he gives general advice to writers and also provides a critique of gothic novels, especially of The Monk by Matthew Gregory Lewis which he considers superior to the work of Ann Radcliffe. One notable story in the collection, Florville and Courval, has itself been called "gothic" and revolves around a young woman who is unwittingly entangled in a web of incest.

While incarcerated again at Charenton, he completed three historical novels: Adelaide of Brunswick, Isabelle of Bavaria and The Marquise de Gange.

He also wrote several plays, most of them unpublished. Le Misanthrope par amour ou Sophie et Desfrancs was accepted by the Comédie-Française in 1790, and Le Comte Oxtiern ou les effets du libertinage was performed at the Theatre Moliere in 1791.

Several letters written from prison to his wife have been preserved and were published in 1998 as Letters from Prison. Some of them show a bizarre and paranoid obsession with the hidden meaning of numbers.

Appraisal and criticism
Numerous artists, especially those concerned with sexuality, have been both repelled and fascinated by de Sade.

Simone de Beauvoir (in her essay Must we burn Sade), published in Les Temps modernes, December, 1951 and January, 1952) and other writers have attempted to locate traces of a radical philosophy of freedom in Sade's writings, preceding that of existentialism by some 150 years. The surrealists admired him as one of their precursors, and Guillaume Apollinaire called him "the freest spirit that has yet existed".

One of the essays in Max Horkheimer and Theodor Adorno's Dialectic of Enlightenment (1947) is titled "Juliette or Enlightenment and Morality" and interprets the behavior of Juliette as the embodiment of the philosophy of enlightenment.

In Harlan Ellison's science fiction anthology, Dangerous Visions (1967), Robert Bloch wrote a story entitled "A Toy For Juliette" whose title character was both named for and used techniques based on Sade's works.

Andrea Dworkin saw Sade as the exemplary woman-hating pornographer, supporting her theory that pornography inevitably leads to violence against women. One chapter of her book Pornography: Men Possessing Women (1979) is devoted to an analysis of Sade. Susie Bright claims that Dworkin's first novel Ice and Fire, which is rife with violence and abuse, is a modern re-telling of Sade's Juliette.