Most underrated evil person in history?

[I spent the winter writing songs about getting better]

Yahya Khan

I'm sure Xahar will be at least somewhat in agreement.

Also Leopold II of Belgium doesn't get a whole lot of attention in comparison to his death toll. You will often see him on "great villains of history" lists, but usually just as an afterthought.

[I spent the winter writing songs about getting better]

Also Fernando Romeo Lucas García. He was probably the worst dictator in the Western Hemisphere in the 20th century, yet virtually no one knows who he was.

[I spent the winter writing songs about getting better]

If you just limit it to recent history, an obvious candidate is Théoneste Bagosora.