The near synonymous nature between Emperor and Big Bad in Anglophone media.
To explain the reason why the title of Emperor is often used only by villainous characters in current English-language media, we have to look at the history of the Anglosphere, which is the current hegemon of popular culture in the Western world, and analyze the interactions it had with the European empires across time that were not the Roman Empire (of which it was a successor state).
First, we have to list the empires that existed in Europe after the fall of Rome. These were the Byzantine Empire, which was the Roman Empire proper albeit weakened; the Carolingian Empire, which only lasted two generations before being split; the Holy Roman Empire, a long runner of almost a millenium when considering Charlemagne as its founder; the Spanish Empire, the first amongst the European countries during the Early Modern Period; the First and Second French Empires, respectively ruled by Napoleon and his nephew Napoleon III; the Austrian/Austro-Hungarian Empire, the Catholic successor of the Holy Roman Empire, ruled by the Habsburgs; the German Empire, the Protestant counterpart of the Habsburgs; the Ottoman Empire, the Muslim master of the Eastern Mediterranean until its demise; and finally, the Russian Empire, the greatest player of Eastern Europe for most of its contemporary history.
Of those empires listed before, the first never interacted properly with England due to the sheer distance between the two countries and the second, as well as the third, held a place of prominence amongst the other European states, due to the dignity conferred by the title as well as the power it theoretically held (there's also the fact that it never waged war against England proper); it's with the Spanish Empire, during the reign of Phillip II, that the Kingdom of England first comes into conflict with a state holding an Imperial rank, something that eventually was mixed both with the various religious wars that marked the Early Modern Period and with the rivalry between France and England after the rise of the House of Bourbon in Spain.
The second Empire that England, now Great Britain, fought against was the First French Empire as an extension of the French Revolutionary Wars that marked the beginning of the Contemporary Era. Most notably, Napoleon attempted an invasion against the United Kingdom itself, stopped only by the tenacity of the British fleet at the Battle of Trafalgar and many other encounters. By this point, the United States had already declared independence, and was looking quietly at the events in Europe.
The Second French Empire, while not properly at war with the USA, did support the Second Mexican Empire, which was seen by the United States government as an illegitimate regime imposed on the Mexican people, a threat to American interests, and a violation of the Monroe Doctrine, which established the United States as the only valid player of influence in the Americas, and which would also be put to the test during the Spanish-American War, which lead to the rise of the USA as a major power.
Back in Europe, Britain only fought one war against the Russian Empire, and that was alongside the French and Ottoman Empires to preserve the balance of power in the continent. However, said balance of power was upset by the rise of the German Empire at the expense of the French one.
Then, the First World War came, which saw all of the Empires of Europe (save for Russia) joined together to fight against Britain, France, the USA and their allies in the debacle of the assassination of Archduke Franz Ferdinand of the Austro-Hungarian Empire.
Now, what do all of these events have in relation to each other? That all of them happened after the invention of the printing press in Europe, allowing the distribution of propaganda against the enemy to spread across like in a manner not seen before. This, coupled with the greater weight of current history in the eyes of media, is perhaps the reason why does the title of emperor evokes a dark image in the Anglosphere outside of historical fiction.
