he wasn't simply a power-mad tyrant like Ivan the Terrible, or a desperate defender of his realm like Vlad the Impaler.

No, he was simply driven by greed and cruelty satisfying that greed.
Imagine Jim Fisk as a king:

Belgium was not trying to be a world power, so was largely ignored.

His sins being in Africa, not Europe or North America, is likely the main reason they are not so well known.

extra information.

Although Leopold was no longer in the picture by then, Belgium also had a pivotal role in one of the bloodiest African genocides of the 20th century.

https://www.pbs.org/wgbh/pages/frontline/shows/rwanda/etc/cron.html

From the link:

And so on and so forth until the inevitable tragedy ensued.

Is king Leopold's Ghost by Adam Hochschild.

Very well written book.

He and Cecil Rhodes were complete travesties to the African continent.

https://www.amazon.com/King-Leopolds-Ghost-Heroism-Colonial/dp/0618001905

"Product description

In the 1880s, as the European powers were carving up Africa, King Leopold II of Belgium seized for himself the vast and mostly unexplored territory surrounding the Congo River. Carrying out a genocidal plundering of the Congo, he looted its rubber, brutalized its people, and ultimately slashed its population by ten million—all the while shrewdly cultivating his reputation as a great humanitarian. Heroic efforts to expose these crimes eventually led to the first great human rights movement of the twentieth century, in which everyone from Mark Twain to the Archbishop of Canterbury participated.King Leopold's Ghost is the haunting account of a megalomaniac of monstrous proportions, a man as cunning, charming, and cruel as any of the great Shakespearean villains. It is also the deeply moving portrait of those who fought Leopold: a brave handful of missionaries, travelers, and young idealists who went to Africa for work or adventure and unexpectedly found themselves witnesses to a holocaust. Adam Hochschild brings this largely untold story alive with the wit and skill of a Barbara Tuchman. Like her, he knows that history often provides a far richer cast of characters than any novelist could invent. Chief among them is Edmund Morel, a young British shipping agent who went on to lead the international crusade against Leopold. Another hero of this tale, the Irish patriot Roger Casement, ended his life on a London gallows. Two courageous black Americans, George Washington Williams and William Sheppard, risked much to bring evidence of the Congo atrocities to the outside world. Sailing into the middle of the story was a young Congo River steamboat officer named Joseph Conrad. And looming above them all, the duplicitous billionaire King Leopold II. With great power and compassion, King Leopold's Ghost will brand the tragedy of the Congo—too long forgotten—onto the conscience of the West.

Review

"An enthralling story, full of fascinating characters, intense drama, high adventure, deceitful manipulations, courageous truth-telling, and splendid moral fervor . . .A work of history that reads like a novel." Christian Science Monitor 

"As Hochschild's brilliant book demonstrates, the great Congo scandal prefigured our own times . . . This book must be read and reread."--Neal Ascherson The Los Angeles Times 

"A vivid, novelistic narrative that makes the reader acutely aware of the magnitude of the horror perpetrated by King Leopold and his minions." The New York Times 

"King Leopold’s Ghost is a remarkable achievement, hugely satisfying on many levels. It overwhelmed me in the way Heart of Darkness did when I first read it—and for precisely the same reasons: as a revelation of the horror that had been hidden in the Congo." -- Paul Theroux 

"Carefully researched and vigorously told, King Leopold’s Ghost does what good history always does -- expands the memory of the human race." The Houston Chronicle

His atrocities should be recognized as on par with Hitler and the North and South American genocide of Native peoples.

Leopold was a long time ago, but if what has been going on in the Congo recently had happened in Europe, it would be called a "World War".