I need to do disclaimers for this one:
First, I am referring to the Marquis de Sade, not the singer Sade. Second, if you feel morbid curiosity striking you and want to read Sade on his own terms, be at least 18 years old as most of his work is porn, and check your country's laws as he's banned in many parts of the world. This infodump will be SFW.
Before we begin, because I am a strong believer in honesty - esp. in criticism - we must acknowledge that Sade is, for better or worse, one of the most influential philosophers in Western history; Jim Morrison, John Waters, Sigmund Freud, Jean-Paul Sartre, Yukio Mishima, Pier Paolo Pasolini, (possibly) Friedrich Nietzsche, Angela Carter, and more, all find influence from his work. However, any critical examination of something, esp. a person's work, should note that influence is not the same thing as quality.
So let's begin with what Sade thought. Firstly, Sade was a materialist philosopher. This shouldn't be confused with being materalistic, just as idealism isn't to be confused with being idealistic. Materalism is a metaphysical philosophy which posits that the world is simply matter. In our modern time this is a lot more common of a take, esp. since physics bore that out quite nicely, but at the time this was still something of a hot take, as idealism was pretty common throughout Europe.
As a consequence of this, however, Sade was also a determinist. In his view, there was no free will, since due to being matter we are all simply acting in accordance with urges provided to us by nature. Following that, he was also a moral/ethical nihilist. Ethics, for Sade, meant basically nothing, since lacking free will also means lacking agency and therefore guilt. This led him to be on the radical left of the French Revolutionary government, despite the fact as a noble from the ancien régime it overthrew, he was liable to the guillotine (though under that regime, he was still liable to the death penalty for other offenses).
OK so now you may ask... what's wrong with this? Well, I left out some things about Sade. The first is that - both by his own admission and by legal record - he was an extremely violent sex pest, and his philosophy was incredibly self-serving to that end. The second is that his determinism is an appeal to nature, which is a logical fallacy. Just as nature gave us desire, it also gave us the capacity to act otherwise. Sade is assuming here that we are simply slaves to desire, and therefore any constraint on that is necessarily social. While social constraints on us do exist, this shouldn't be confused for "Oh well, I want to do something extremely depraved, I guess I have no choice".
Furthermore, Sade projects a bit; Sure, we have urges which are not socially acceptable. They may even be cruel urges. But we also have perfectly mundane urges. Hunger is allotted to us by nature, as is thirst, and tiredness. There is nothing inherently vile in these urges.
So then, the question should arise - why is Sade so influential? The answer is multifaceted, and depends upon who the influenced is. Sade prefigures Freud in asserting that pleasure is the primary drive in humans. His poetry and violent eroticism influenced the writing style of Mishima. Speaking of said eroticism - extreme and unorthodox though it was - Sade's openness in many ways prefigures sex positivity and the Sexual Revolution. It is from his name that the term "sadism" (albeit originally as a clinical diagnosis) emerges. The list goes on.
Ultimately, for better or worse, Sade is one of the most influential thinkers and writers in the history of the West. But even the influential deserve some level of skepticism.
