It seems that Hitler quickly became the prototypical bad guy in history. The go to example that people use for a maleficent ruler (and ultimate internet insult). Who would be the closest to the unanimous villain before WW2? Whom would a turn of the century teenager compare you to make their point?
This question has been asked before, and attracted enough worthwhile answers to form part of the FAQ:
Short answer, Judas. There is an existing thread that answers similar question: What was 'the worst thing ever' before Hitler and the Holocaust? What example would Godwin have used, if he lived at different times in history?
If we narrow down to just maleficent rulers, there is no real equivalent. Lets take for example Attila, the Scourge of God. He may be hated in Roman or Christian sources, but is used as venerated ancestor in the "Notary of King Béla" or "Nominalia of the Bulgarian khans". When the Constantinople fell, Pope Calixtus III did not call Mehmed the Conqueror new Attila, "Bulla Turcorum" instead used biblical references such as Sennacherib and Nebuchadnezzar.