For the time period and what he was able to do I think Attila the Hun was the worst tyrant. Could you imagine what he would do if he had the mechanics of World War II?
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Genghis Khan - he's alleged to have killed 40 million people, directly or indirectly. Entire cities and kingdoms were wiped out. Mao had 20th century technology and got about the same score. Imagine what Genghis with machinegun wielding Huns could have done.
What about Alexander the 'Great'. He may sound great for western cultures but for everyone in Asia that was subjected to his men he wasn't that way.
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I would go with Hitler or Stalin. What Genghis Khan and Alexander the Great did was normal in that time period. They conquered other lands and subjugated their people. They just did it on a larger scale. Hitler and Stalin brutally murdered millions of their own people for various reasons. That isn't even comparable to what Genghis and Alexander did.
Alexander and Genghis Khan didn't set out to murder people. They wanted to expand their empires. This was common of most rulers in the past.
Edited: Kyrroeth on 15th Oct, 2017 - 1:29pm
If you look at Hitler or Stalin as the worse I would have to go with Stalin as the worse. He did it for no other reason than the fact that he could do it.
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I spent like 5 minutes contemplating this before answering and I decided I want to answer it a bit differently than my initial plan. At first I was going to try to figure out which human has caused the most suffering and death and call that person the worst tyrant. However I've decided to use the other interpretation of "Worst" as in who sucked at being a tyrant the most.
In that sense, I'd say the worst tyrant might have been Caligula. With all the power he had back then he couldn't even protect himself from the people in his house from stabbing him in the back. What a failure of a tyrant, he spent too much time following those sexual pursuits rather than paying attention to the political situation and emotional states of the people he hung around.
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