From reading Sade's text, i often thought that Sade had well-earned him the reputation as the most psychotic figure in the revolution, given his advocacy for rape and other sexual crimes.
And yet, i've seen some accounts that his books are merely meant to be a commentary on humanity and a satire of French society at the time, specially the nobility, and Sade was simply commenting and criticizing the attitudes of the community that he lived in, possibly even showcasing it's depravity as a form of condemmation given that he was against the death penalty (even if he had earned a genuine reputation as a Casanova)
And yet i've seen other people give descriptions of him drugging prostitutes and sexually abusing his servants and coercing them into having sex with him. Overall, i've seen many sources give different opinions on Sade and many people who regard him either as a sexual revolutionary or as a predator who barely contained his urges. So, which account is true?
Although I certainly won't qualify as a historian, I might be able to give some leads on sources. I'm a bachelor student in philosophy, and I have written a short piece on De Sade for a magazine. In his books De Sade both writes sexual perverse scenes and progressive thought. Whilst the examples of sexual perversion are more easy to find, I shall give some short examples of more progressive thought.
What's most clearly to define, is Sade's atheism. In Aline et Valcour he criticizes the way in which religiosity seems to be chosen in favour of the actual lives of people: to be able to accompany a dying man, a pastor allows himself to take a shortcut through a cornfield, and in doing so he destroys the crops of people who need it. The heroes of the story intervene, tell the priest that is 'good deed' was merely a contrivence - only something useful can be an act of good. In Dialogue entre un Ptêtre et un Moribond (English: Dialogue Between a Priest and a Dying Man) De Sade also writes against the mistakes of living a pious life. However, his atheism isn't the most interesting element of De Sade's thought and life.
In La Philosophie dans le Boudoir he writes against the death penalty, but he doesn't seem to be against violence or murder on a personal level. The inconsequential result of the death penalty, which is a punishment for murder, is two deaths instead of one, which is satisfying for "rascals and fools".^1 However, throughout his works, he clearly believes one should be able to follow nature. If a part of you has the urge to murder someone, following De Sade's reasoning one should be allowed to do so.
When it comes to how to treat the poor, being inconsequential is also one of his points of critique. You should either strive for equality and help the poor, or simply kill them. This also comes back in his thought on theft, which he also writes about in La Philosophie dans le Boudoir, but similarly in Les Infortunes de la Vertu. According to De Sade, the poor should be allowed to steal, because that gives them the chance to become more equal to the rich. If you want to prevent this, you should improve equality.
Sexually, De Sade is more difficult to grasp. He often writes about sodomy, and he doesn't seem to be bothered with any heteronormative thought. Multiple of his characters favour homosexual acts over heterosexual acts, which could be seen as progressive. Women often get treated violently, even mutilated. Yet, Angela Carter, in her book The Sadeian Woman and the Ideology of Pornography states that it was progressive of De Sade to see women as a sexual creature, with lusts and wishes of their own, instead of mere 'breeding machines'. (Once again, I am not a historian, so I can't adequatly reflect on how progressive this was during De Sade's own time.) Furthermore, in a piece from 1980 by A.D. Farr, The Marquis de Sade and induced abortion., Farr states that:
[De Sade] proposed the use of induced abortion for social reasons and as a means of population control. It is from this time that medical and social acceptance of abortion can be dated, although previously the subject had not been discussed in public in modern times. It is suggested that it was largely due to de Sade's writing that induced abortion received the impetus which resulted in its subsequent spread in western society.
Biographically and socially, an interesting source might be Simone de Beauvoir's Faut-il brûler Sade? (English: Must We Burn Sade?).^2 In this work she states that his legacy has been mutilated by "stupid legends", which made him more evil than he actually was. He was supposedly similar to more young aristocrats of his time - slowly losing power, trying to symbolise the power they were 'supposed' to have in the bedroom. De Beauvoir states that De Sade might been a bit of a coward, she compares him to Blangis in Les Cent Vingt Journées de Sodome, who could have been scared by a child. He flees in his eroticism, because that is the only way to "furfil his existence". However, even if this is fully true, that doesn't make his deeds less cruel.
De Sade also seems to have had an actual criminal (or evil, if you will) side. Throughout his life, he has spent of twenty years in prison. I'll shortly list some of his crimes as reported in the Dutch translation of Faut-il brûler Sade?. In 1768, he abuses a 36-year old widow, Rose Keller, and has to appear in front of court. , the abuse seems to be of a sexual nature, but doesn't involve penetrative, oral or manual sex. In 1772, De Sade is accused of unruly behaviour with four prostitutes, and later with a young woman, who accuse him of poisoning them. He appears not to have done so. In 1775 he is accused of the temptation and rape of young people.
So, to answer your question in short: both are true. One is able to see him as progressive in his writing, but he was also a predator in (some of) his behaviour. Something that I haven't read enough of to be able to use it as a source, is Jean-Jacques Pauvert's biography of De Sade, in which he wants to de-demonize De Sade, by telling his actual story. The parts that I've read were very interesting, so perhaps that might be a good point for future research.
I hope I've been helpful!
^1 Translated to English from a Dutch translation.
^2 Again, I am using a Dutch translation, some words might be different from how they're used in French or in an English translation.
You may find this excellent answer by u/AnnalsPornographie from a few years back gets at some of your question!