Who are the most depraved and brutal rulers pre 1900?
Depraved: marked by corruption or evil; perverted
I’m talking rulers who displayed little to no regard for human life, sadistic tendencies, mental instability, psychosis, psychopathy, or any combination thereof.
Also rulers who lived decadent lifestyles equivalent to the cocaine fueled orgies of our time.
So many worthy candidates.
I’ll get the ball rolling with King Leopold II of Belgium.*
“Leopold amassed a huge personal fortune by exploiting the natural resources of the Congo. At first, ivory was exported, but this did not yield the expected levels of revenue. When the global demand for rubber exploded, attention shifted to the labor-intensive collection of sap from rubber plants. Abandoning the promises of the Berlin Conference in the late 1890s, the Free State government restricted foreign access and extorted forced labor from the natives. Abuses, especially in the rubber industry, included the effective enslavement of the native population, beatings, widespread killing, and frequent mutilation when the production quotas were not met…Since the first official census by the Belgian authorities in 1924 put the population at about 10 million…various approaches suggest a rough estimate of a total of 10 million dead.”
*there’s some overlap of pre- and post-1900 for the worst atrocities.
An obscure favorite Queen Ranavolana of Madagascar
The Roman emperor Caligula probably falls into this category.
There was this one guy in late 14th century…damnit, name is right on the tip of my tongue…arghh…I just can’t seem to recall…
But that unnamed jerk aside :), there was an awful lot of violent sadism in the pre-modern era. Some other guy who sacked Delhi…like…whoever that was…was Nadir Shah. Quite brilliant military leader and had a few oddly endearing habits for a bloody-handed usurper and tyrant ( he was raised by his widowed mother and before he went nuts he was a bit more chivalric towards women than was typical for his time and place ). But for whatever reasons he steadily went insane in his last years and committed paranoid and vicious barbarities by the truckload until he was assassinated.
Thomas of Marle…Robert of Belleme…Arnaud-Amalric of Citeaux…Mehmed the Conqueror and his sometime opponent Vlad Tepes. Lots o’ bad guys. Even if in some cases some of their transgressions were perhaps more alleged by folks with axes to grind than proveable.
Let’s get two obvious contenders out of the way : Elizabeth Bathory who had a thriving business in virgin procurement ; and Gilles de Rais who was, by some accounts, one of the first* bona fide* serial thrill killers and certainly one of the most prolific - he’s credited with anywhere between 50 and 600 dead kids.
Queen Fredegund killed and tortured everyone who stood in her way or looked at her funny, even the clergy. She killed those who failed her, making her cartoonishly vindictive. She almost killed her daughter by repeatedly slamming a treasure chest lid against her head, until her servants restrained her. She’s often cited as the inspiration for Cinderella’s step mother. She makes Cersei look like a girl scout.
She’s obscure in the wider culture, but she’ll pop up in any discussion of badass/cruel women. My favorite story about her is that if she wanted to kill some pesky diplomats or foreign officials while maintaining plausible deniability she’d have her handlers take them on a scenic route through the jungle. Most of them would die of some exotic illness.
Queen Nzinga is another. A lot of the more extreme stories are probably propaganda and tall tales from both sides, but if you believe everything you read then she chose which of her male harem to have sex with by making them fight to the death. Before battle she would slit the throat of a slave and drink his blood. She ate the hearts of her enemies to gain their power. Then again, if you believe that stuff works and you’re leading a heroic campaign for your country, is it cruel? The Aztecs did horrible things, but they thought if they didn’t the world would end.
Wu Zetian was another powerful queen fond of murder. There’s always arguments over whether she killed her baby to frame her rival or not.
William of Normandy
Timur the Lame, better known as Tamerlane, conquered and/or invaded a great deal of the known world in 15th Century. Some scholars estimate that his campaigns led to the death of about 5% of the current world’s population (about 17 million deaths).
I was thinking of the same hard-to-remember fellow.
Another African favorite is Shaka Zulu - though historians now dispute just how horrible he was, I think the better view is that he was pretty horrible.
Vlad the Impaler.
If we are talking China, then you just can’t beat Qin Shi Huang, First Emperor - pure selfishness taken to a monsterous level - an Orwellian figure, he allegedly had all books (save on agriculture) burned, buried scholars alive, and thousands died in his forced-labour projects - including building his elaborate tomb. Goes above and beyond your average mass-murdering warlord.
Halagu Khan has Timmy beat.
I think the OP wanted personal, hands-on depravity rather than hands-off brutality by the minions.
The Death rumor of Catherine the Great has been dis-proven by Snopes.
Her exploits while alive, which seem similar, have not.
Do you mean personal depravity, or enacting depraved policies ?
If you mean policies I would go for one of the Mongols rulers, because the Mongols wiped out a large percentage of the human race, or maybe some of the rulers that claimed the Americas (north or south), given the scale and brutality of the conquest.
I’ve seen the Mongol body count given as high as 75,000,000, and that includes every type of rape, torture and pillage along the way.
What’s one Bagdad more or less between friends?
Anyway, he was a fastidious sort - he never shed royal blood … 
The conquest of the Americas gets marks taken off, because much of the death was caused by introduced disease - plus, not really the work of any one person. Though guys like Cortez and Pizarro were hardly nice, their acts (and those of others) created death on a scale they could not really have anticipated.
She had a hearty appetite for Russian noblemen - apparently she had her secretary try them out first as a sort of audition - but not (alas) for horses.